Preheat the oven to 350°F (175°C).
Cook the egg noodles according to package instructions until al dente. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add the diced onion and cook for 4-5 minutes until softened.
Add the minced garlic and sliced mushrooms to the skillet. Cook for an additional 5-6 minutes until the mushrooms release their moisture and become tender.
Sprinkle the flour over the vegetables in the skillet and stir well to coat. Cook for 1-2 minutes to remove the raw flour taste.
Gradually whisk in the chicken broth and almond milk, stirring constantly to avoid lumps. Bring the mixture to a simmer and cook until it thickens, about 3-4 minutes.
Stir in the Dijon mustard, dried thyme, salt, and black pepper.
Remove the skillet from heat and gently fold in the drained tuna and thawed peas.
Combine the cooked noodles with the tuna mixture and stir until evenly mixed.
Transfer the mixture to a 9×13-inch baking dish and spread evenly.
In a small bowl, mix the panko breadcrumbs with chopped fresh parsley. Sprinkle the breadcrumb mixture evenly over the casserole.
Bake in the preheated oven for 20-25 minutes until the top is golden brown and the casserole is heated through.
Remove from the oven and let cool for 5 minutes before serving.