Cook the elbow macaroni 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 cook for 3-4 minutes until softened.
Add the minced garlic and sliced mushrooms to the skillet. Cook for another 4-5 minutes until mushrooms are tender.
Sprinkle the all-purpose flour over the vegetables in the skillet. Stir constantly for 1-2 minutes to cook the flour and create a roux.
Gradually whisk in the whole milk and chicken broth, stirring continuously to avoid lumps.
Bring the mixture to a simmer and cook for 5 minutes until the sauce thickens.
Remove the skillet from heat and stir in the shredded cheddar cheese until melted and smooth.
Season the sauce with salt, black pepper, and dried thyme.
Add the cooked macaroni, thawed peas, and flaked tuna to the skillet. Gently fold to combine all ingredients evenly.
In a small bowl, mix the breadcrumbs, grated Parmesan cheese, and olive oil until the crumbs are evenly coated.
Sprinkle the breadcrumb mixture evenly over the top of the casserole in the skillet.
Place the skillet under a preheated broiler for 3-5 minutes or until the breadcrumb topping is golden brown and crispy. Watch carefully to prevent burning.
Remove from the oven and let the casserole cool for 2 minutes before serving.