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Sprinkle the remaining 1/2 cup shredded cheddar cheese evenly over the top. This will give you a nicely browned, bubbly top layer, just like the old-fashioned casseroles we all grew up with.
Place the glass baking dish on the middle rack of the preheated oven. Bake for 25–30 minutes, or until the casserole is set in the center (it should not be jiggly when you tap the dish) and the top is lightly browned and bubbling around the edges.
Remove the casserole from the oven and let it rest for about 5–10 minutes before serving. This short rest helps it firm up a bit so it scoops out in nice, hearty portions.
Spoon into bowls or onto plates and serve warm. Store any leftovers, cooled, covered in the refrigerator and use within 3 days.
Variations & Tips
If you’d like a little more vegetable bulk while keeping things low carb, you can stir 1–2 cups of well-drained, cooked broccoli florets or riced cauliflower into the tuna before you pour on the egg mixture. A teaspoon of dried dill or Italian seasoning mixed into the egg and sour cream will give it a different twist, and you can swap part of the cheddar for Monterey Jack, pepper jack, or Swiss if that’s what you have. For extra richness, replace a few spoonfuls of sour cream with heavy cream or cream cheese. If your household likes a bit of heat, add a pinch of red pepper flakes or a few dashes of hot sauce to the egg mixture. For food safety, always use tuna from cans that are not bulging, rusted, or badly dented, and refrigerate leftovers within 2 hours of baking. Reheat leftovers until they are hot all the way through (165°F/74°C) before serving, and discard any casserole that has been left out at room temperature for more than 2 hours. If using a glass baking dish, avoid placing it directly from the refrigerator into a very hot oven to prevent thermal shock—let it sit at room temperature for a short while before reheating.
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