Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish with butter or nonstick spray and set aside.

Bring a large pot of salted water to a boil. Cook the egg noodles 2 minutes less than the package directions (they will finish cooking in the oven). Drain well and set aside.

In a large skillet or saucepan over medium heat, melt 3 tablespoons of butter. Add the diced onion and cook, stirring occasionally, for 4–5 minutes until softened and translucent. Add the minced garlic and cook for 1 more minute until fragrant.

Sprinkle the flour over the onion mixture and stir constantly for 1–2 minutes to cook out the raw flour taste.

Slowly pour in the warmed milk and chicken broth, whisking constantly to prevent lumps. Cook over medium heat, stirring frequently, for 5–6 minutes until the sauce thickens enough to coat the back of a spoon.

Reduce heat to low. Stir in 1 cup of the shredded cheddar, Dijon mustard, garlic powder, onion powder, salt, pepper, and lemon juice. Stir until the cheese is fully melted and the sauce is smooth. Taste and adjust seasoning.

Add the drained egg noodles, drained tuna, and frozen peas to the sauce. Gently fold everything together until evenly combined. Transfer the mixture to the prepared baking dish and spread into an even layer.

Sprinkle the remaining 0.5 cup of shredded cheddar evenly over the top.

In a small bowl, combine the panko breadcrumbs, melted butter, and grated parmesan. Stir until the crumbs are evenly coated. Sprinkle the breadcrumb mixture evenly over the top of the casserole.

Bake uncovered at 375°F for 25–30 minutes until the casserole is bubbling around the edges and the breadcrumb topping is deep golden brown.

Remove from the oven and let rest for 5 minutes. Garnish with fresh chopped parsley and serve hot.