Corn, milk, and soybeans are the biggest agricultural products by weight in the US.
So maybe some tofu tacos with cheese, or quesadillas with tofu? 
@ucbalumnus, I wouldn’t serve tofu to somebody, who by the Op’s description is a picky eater.
I am not a picky eater and don’t eat tofu!
This sounds like my son. He was fairly picky right up until his freshman year of college. He liked baked chicken, london broil, pizza, boneless ribs, meatloaf, baked fish, clam cakes, grilled cheese, ham, and strombolis. He’d eat a baked potato but preferred baked wedges. Steamed broccoli and carrots were okay, but raw was better. Corn only tasted good on the cob, and salad was good as long as it didn’t contain olives or dressing. He’d eat french toast or pancakes and bacon for dinner, but the only eggs he’d eat had to be scrambled and hidden between the slices of a huge Italian roll. Now he eats just about anything that doesn’t move.
How about chicken Parmesan?
Mix one part Italian breadcrumbs with 1 part Parmesan cheese (some like more breadcrumbs, some like more cheese)
Add a little dried parsley flakes
Melt a stick of butter
Dip boneless chicken breast in the butter, then into the breadcrumb mixture
Put into a 13x9x9 baking pan
Once you’ve got all the chicken in the pan, just drizzle rest of butter over the top
Bake on 350 for about an hour
It’s kind of boring, but that sounds like what you’re looking for.
Some people serve with pasta and red sauce, but that’s totally optional and I don’t.
How about a roast chicken like the kind sold at Costco or many supermarkets? you can make mashed potatoes to go with it - my H is a mashed potato lover but he will eat the powdered baby red kind that you cook in boiling water.
The chicken is good because different people like different parts - middle son and I like the dark meat and everyone else likes white meat.
A couple of these suggestions sound really good to my adventurous palate, but for a family of “picky” eaters who don’t like anything remotely “foreign” tasting? I’m baffled.
Yeah, I love leeks, but that suggestion would not go over will with my picky family members.
You’ve received quite a range of suggestions. I’m sure some of them would work out great. Good luck! Your family members are lucky you’re putting such great thought and consideration into your contributions.
Oven roasted or mashed potatoes are a good alternative if they’ve been eating a lot of pasta lately. Serve with meatloaf (great suggestion) or roasted chicken and veg.
My ex-SIL’s mother used to make a wonderful, homey chicken-broccoli-rice-cheddar casserole as her go-to dish for potlucks or family support dinners. Simple flavors and nothing challenging in it.
It would be a kindness (since these seem to be people you like) to refer to them as “selective” eaters rather than “picky” eaters.
Picky pretty much sums it up, though.
Selective to me refers to quality, picky refers to being very limited in the number of foods one eats. 
@Wellspring, I do like them. I love them! But they are picky eaters for sure (one person especially). It’s not a case of “Which excellent thing can I choose today?” but rather “Oh, man, gotta eat again? Hope it’s nothing with texture or taste that offends me, 'cause then I just won’t.”
I meant no offense in choosing my words! But please, let’s not get sidetracked. I’m really appreciating all these thoughts about plain food and will save them for other times when I’m going to feed this fam.
Scalloped potatoes and ham. Meatlof and mashed potatoes-homemade. Beef stew. Beef Stroganoff. Barbecue ribs. Corn casserole. Green bean casserole. Some folks just overwhelm quickly with new tastes and at time of stress, less tolerant of anything out of the norm.
Well, tofu is pretty bland, as are corn tortillas and some kinds of cheese. The person’s preferred sauces or seasonings can be added, of course. I doubt that the embedded government subsidies from their tax dollars can be tasted. 
Perhaps a different idea is roast turkey, Thanksgiving-style.
There are other ideas listed at https://en.wikipedia.org/wiki/List_of_American_foods .
I have to agree with @CottonTales . I am also a picky eater and I can’t stand the touch of tofu against my tongue. It’s not the blandness, it’s the texture for me.
Lol, “plain America food.” They “don’t like anything spicy, no tomato sauce, nothing foreign-seeming.” You could probably fool them somehow on the tofu, but not the corn tortillas. And remember, one has dexterity problems.
But turkey dinner sounds great.
LOL. Leeks are just onions dressed up. Anything that calls for leeks can have onions instead. But yes, maybe they should just have roast chicken, plain pork roast (maybe with a little apple sauce?), and brisket. There’s nothing wrong with any of that.
I am totally the person in the OP… or at least, 10 years ago. I will eat marinara now . Mr R is still the person in the OP except he eats meat and I don’t.
Do you have to make a traditional main dish? Most picky eaters I know, including myself, love the sides. That way we can pick and choose what we want.
Here you go, @mathmom.
https://www.youtube.com/watch?v=pQWw0UrSGJw
Some more ideas for American food:
https://en.wikipedia.org/wiki/Native_American_cuisine
Since maize/corn is native to the Americas, and is by far the largest agricultural product in the US, here are some maize/corn-based foods:
https://en.wikipedia.org/wiki/List_of_maize_dishes