Preheat the oven to 375°F and lightly grease a 9x13 baking dish.

Cook the egg noodles in a large pot of boiling water until just tender, then drain and set aside.

In a skillet over medium heat, cook the diced onion until soft, about 3 minutes.

Stir in the cream of mushroom soup, milk, salt, and black pepper until the mixture is smooth.

Add the drained tuna, frozen peas, and shredded cheddar cheese to the skillet and stir to combine.

Fold the cooked noodles into the sauce mixture until everything is evenly coated.

Transfer the mixture to the prepared baking dish and spread it into an even layer.

In a small bowl, toss the breadcrumbs with the melted butter, then sprinkle the mixture evenly over the top.

Bake for 20 to 25 minutes until the topping is golden and the casserole is bubbling at the edges.