Preheat the oven to 350°F (175°C).
Cook the egg noodles in a large pot of boiling salted water according to package instructions until al dente, about 7-8 minutes. Drain and set aside.
In a large skillet, melt the butter over medium heat. Add the chopped onion and cook until softened, about 4 minutes.
Add the minced garlic and cook for 1 minute more until fragrant.
Sprinkle the flour over the onion and garlic mixture and stir constantly for 1-2 minutes to cook the flour.
Gradually whisk in the milk and chicken broth, stirring constantly to avoid lumps.
Add the Dijon mustard, salt, black pepper, and dried thyme. Continue to cook, stirring frequently, until the sauce thickens, about 5 minutes.
Remove the sauce from heat and stir in the flaked tuna and thawed peas.
In a large mixing bowl, combine the cooked noodles with the tuna sauce mixture. Stir in the shredded cheddar cheese.
Transfer the mixture to a greased 9×13-inch baking dish and spread evenly.
In a small bowl, mix the breadcrumbs with the grated Parmesan cheese. Sprinkle this evenly over the casserole.
Bake uncovered in the preheated oven for 20-25 minutes, or until the top is golden brown and the casserole is bubbly.
Remove from the oven and let rest for 5 minutes before serving.