Preheat oven to 375F. Lightly spray a 9x12 casserole dish with cooking spray.

Cook noodles in salted water until al dente, or slightly undercooked by 2 minutes. Set aside.

Melt the butter in a large deep skillet. Add onions and cook on medium heat until soft, about 5 minutes.

Add the flour and a pinch of salt and stir well, cooking for an additional 2-3 minutes on medium-low heat.

Slowly whisk in the broth until well combined, increasing heat to medium and whisking well for 30 seconds, then add the milk and bring to a boil.

When boiling, add sherry, mushrooms, and petite peas. Add salt and black pepper to taste and simmer on medium, mixing occasionally until it thickens (about 7 to 9 minutes). Add drained tuna, stirring for another minute.

Remove from heat and add the reduced fat sharp cheddar, mixing well until it melts. Add the noodles to the sauce and mix well until evenly coated. Pour into the casserole dish and sprinkle the top with Parmesan cheese and breadcrumbs. Spray a little more cooking spray on top and bake for about 25 minutes.

Place under the broiler for a few minutes to get the crumbs crisp (be careful not to burn).