Preheat oven to 375°F; cook noodles al dente, then drain.
Heat oil in skillet; sauté onion 3-4 min, garlic 1 min, mushrooms 5-6 min.
Add butter, flour; whisk in milk, broth. Cook until thickened. Add peas, 1/2 cup cheese, salt, pepper, and thyme. Taste and adjust the seasoning.
Mix noodles, tuna, and sauce; transfer to a greased 9x13-inch white ceramic baking dish.
Top with 1/2 cup cheese, chips, or breadcrumbs. Bake 20-25 min.
Cool 5 min; serve in a baking dish with parsley, salad, and iced tea.