It's basically an open-faced sandwich
Thick-sliced bread
Ham or hamburger patties
French fries
Cheese Sauce
Now from that you can sub any of various meats (commonly deep fried pork tenderloin, grilled or fried chicken breast, and fried fish filets or eggs), various potatoes (steak fries, homestyle, hashbrowns, etc.) and the cheese sauce will vary from place to place too (Cheddar, white sharp Cheddar).
Here's a list of some Springfield, IL restaurants that serve them:
Barrel Head Pub D’Arcy’s Pint Delaney’s Dublin Pub Ritz’s Sammy’s Sportsman’s Lounge West Woods If you gather from their menus you see a compilation of:
Bread layer:
Texas Toast
Meat Layer:
Ground Sirloin, roast beef, bacon & tomato, grilled buffalo chicken, breaded buffalo chicken, corned beef, pork chop, turkey breast, ham, grilled chicken, batter dipped cod, chili, 1/2 ham & 1/2 turkey, breaded pork tenderloin, veggie, hamburger, Italian sausage, Walleye, ground beef with onions & bacon, chili cheeseburger, chili cheese hot dog, egg, bacon, veggie burger, shrimp, fried chicken strips, pot roast, Philly beefsteak
Potato layer:
Steak fries, homestyle fries, french fries, crinkle cut fries, hashbrowns, onion rings, tater tots, potato chips, deep-fried round potatoes, tortilla chips,
Cheese sauce layer:
Yellow cheese sauce, spicy cheese sauce, traditional cheese sauce, white cheese sauce, sausage gravy, Hollandaise, Blue cheese sauce, rarebit cheese sauce
Here's a good read on them:
http://whatscookingameric.../HorseshoeSandwich.htm