Remove the casserole from the oven and let it rest for about 5–10 minutes before serving. This short rest helps the sauce thicken slightly and makes it easier to scoop neat portions.
Spoon the savory French onion noodles onto plates or into bowls, making sure each serving gets some of the creamy sauce from the bottom of the dish. Serve warm.
For a heartier version, stir 1–2 cups of cooked, shredded chicken or diced ham into the noodles before pouring on the soup mixture. If you like extra onion flavor, add 1/2 teaspoon onion powder or a handful of finely chopped sautéed onions to the soup and sour cream mixture. To stretch the dish for a crowd, you can add up to 1/2 cup of milk or cream to the soup mixture for a bit more sauce, adjusting the baking time by about 5 minutes if needed. Swap Swiss cheese for mozzarella, provolone, or a mild cheddar if that’s what you have on hand; just avoid very sharp cheeses that can overpower the gentle onion flavor. For a little crunch, uncover the casserole for the last 10 minutes of baking and sprinkle 1/2 cup crushed butter crackers or French-fried onions over the top. If you prefer a lighter version, use light sour cream and reduce the cheese to 3/4 cup, understanding it will be a bit less rich but still comforting. Leftovers reheat nicely in a covered dish at 325°F with a splash of milk or broth stirred in to loosen the sauce.
