Preheat the oven to 350°F (175°C).
Cook the egg noodles in a large pot of salted boiling water according to package instructions until al dente. Drain and set aside.
In a large skillet, melt the butter over medium heat. Add the chopped onion and sauté for 3-4 minutes until softened.
Add the minced garlic to the skillet and cook for 1 minute until fragrant.
Stir in the frozen peas and cook for another 2 minutes until heated through.
In a large mixing bowl, combine the cream of mushroom soup, milk, and shredded cheddar cheese. Stir until smooth.
Add the cooked noodles, sautéed onion, garlic, peas, and flaked tuna to the bowl. Season with salt and black pepper to taste. Mix everything gently until well combined.
Transfer the mixture to a 9×13-inch baking dish and spread evenly.
In a small bowl, combine the dry breadcrumbs and grated Parmesan cheese. Sprinkle this mixture evenly over the casserole.
Bake uncovered in the preheated oven for 25-30 minutes or until the casserole is bubbly and the topping is golden brown.
Remove from the oven and let it cool for 5 minutes before serving.