Cook the pasta according to package directions. Drain and set aside.
Spray the inside of an oven safe ramekin or small baking dish with non-stick cooking spray. Set aside.
Melt the butter in a small saucepan set to medium heat. Add the onions and stir until softened, about 4 minutes.
Stir in the flour, salt, pepper, nutmeg, thyme, and garlic powder to the pot and cook, stirring constantly for 1 minute.
Whisk in the milk, cream, and lemon juice and simmer sauce, whisking occasionally, until thickened, about 5 minutes.
Remove from heat and stir in cheeses.
Put the tuna in a large bowl, stir in the sauce and the cooked pasta. Mix well.
Pour into the prepared ramekin.
In a small bowl, mix together the bread crumbs and the melted butter. Sprinkle over top of tuna casserole.
Bake in the oven for 20-25 minutes or until topping is golden.
Notes
Pasta Choices: While I use small elbow macaroni, feel free to swap it out. Small-shaped pasta like penne or fusilli works well, and you can go for regular, whole wheat, or gluten-free varieties to suit your dietary needs.
Veggie Boost: If you enjoy a touch of green in your casserole, toss in 1/2 cup of frozen peas while the sauce simmers. It's an easy way to add color and nutrients.
Cheese Variations: While sharp cheddar is my go-to, consider mixing it up with smoked Gruyère, fontina, or sharp white cheddar for a unique twist on flavor.