To make this casserole gluten-free, swap out the all-purpose flour in the sauce for a gluten-free flour blend or cornstarch (just use a little less if using cornstarch). And check your French fried onions; some brands contain gluten, but there are gluten-free options available! With these small changes, you’ll have a comforting, gluten-free casserole that’s just as creamy, delicious, and packed with flavor as the original.