Step 1: Preheat Oven & Prepare Dish – Preheat your oven to 375°F (190°C). Butter a 3-quart (9x13 inch or similar) baking dish.
Step 2: Cook the noodles – Cook egg noodles according to package directions, reducing cooking time by 1 minute. Reserve 1 cup of the starchy cooking water. Drain the rest off and set the noodles aside.
Step 3: Saute the aromatics – In a large skillet, melt 3 tablespoons of butter, and saute shallots (or onions), and celery for 5 to 7 minutes until translucent and soft.
Step 4: Add seasonings and thickener – Add garlic powder, thyme, salt and black pepper. Sprinkle in all-purpose flour and cook for 2 minutes.
Step 5: Finish the sauce – Gradually whisk in milk and reserved starchy water. Cook on medium heat for 5-10 minutes or until the sauce is thickened and coats the back of a spoon. Remove the pan from heat and stir in lemon juice and shredded cheese. Stir until melted and smooth.
Step 6: Assemble the casserole – In a large mixing bowl, fold in noodles, drained tuna, peas, cheesy bechamel sauce, and mayonnaise. Gently fold until evenly combined. Transfer the tuna mixture to the prepared baking dish and spread evenly.
Step 7: Prepare the topping – Combine melted butter and bread crumbs. Top the casserole with the buttered breadcrumb mixture.
Step 8: Bake – Bake uncovered for 25-30 minutes on the middle rack of oven until bubbly and golden brown on top. Serve and enjoy! Let rest for 5 minutes before serving. This allows the sauce to thicken slightly and makes it easier to scoop without falling apart.