Cook noodles for slightly less time than package instructions so they are al dente (firm to the bite). Drain well.
While noodles are cooking, melt the 1/4 cup of butter in a large saucepan over medium heat. Whisk in flour until combined. Slowly add a cup of hot milk whisking constantly until the mixture is smooth and starts to bubble. Continue adding hot milk and whisking until all the milk has been added and the mixture is smooth. Bring to a simmer and cook for 5 minutes, whisking often.
Remove sauce from heat and whisk in mushroom soup, 1 tsp salt and 1/4 tsp pepper. Stir in half of the cheddar cheese, tuna, and peas until combined. Add drained noodles and stir to evenly coat.
Spray a 9"x13" casserole dish with non-stick spray and pour casserole into the dish, spreading out evenly. Top with remaining cheddar cheese.
Stir together breadcrumbs, melted butter, parsley and a sprinkle of salt and pepper. Spread evenly over casserole and bake on the centre rack for 35-40 minutes until bubbly and the top is golden. Remove from oven and serve immediately.