Preheat the oven to 350°F (175°C). Lightly grease a 9×13 inch casserole dish.
Cook the egg noodles in a large pot of boiling salted water according to package instructions until al dente. Drain and set aside.
In a large skillet, melt the butter over medium heat. Add the sliced mushrooms and cook until they release their moisture and begin to brown, about 6-7 minutes.
Add the chopped onion and cook until softened, about 4 minutes. Stir in the minced garlic and cook for 1 minute until fragrant.
Sprinkle the flour over the mushroom mixture and stir constantly for 2 minutes to cook the flour.
Gradually whisk in the milk and chicken broth, continuing to stir until the sauce thickens, about 5 minutes.
Season the sauce with salt, black pepper, and dried thyme. Remove from heat.
Stir in the thawed peas, flaked tuna, and shredded cheddar cheese until evenly combined.
Add the cooked egg noodles to the sauce mixture and gently fold to combine.
Transfer the mixture to the prepared casserole dish and spread evenly.
In a small bowl, toss the breadcrumbs with olive oil until evenly coated. Sprinkle the breadcrumb mixture evenly over the casserole.
Bake uncovered in the preheated oven for 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.