Spray a 9x13 baking dish with cooking spray. Set aside.
Cook egg noodles in a large pot, according to package instructions. Strain and set aside.
Add 4 tbsp of butter to a large skillet with diced onion. Cook on medium-high until translucent.
Add flour and whisk until a thick paste forms. Slowly add chicken broth and then milk, whisking until smooth and creamy.
Mix in italian seasoning, nutmeg, and lemon juice.
Mix in turkey, mushrooms, peas, and cheeses.
Season with salt and pepper to taste.
Add in cooked egg noodles and gently mix to combine.
Transfer to a casserole dish and smooth into an even layer.
In another saute pan, melt remaining 4 tbsp butter over medium-high heat. Add breadcrumbs and cook until golden, stirring constantly. Remove from pan immediately so they don't continue to brown in the hot pan. Sprinkle breadcrumbs evenly over top of the casserole.
Bake for about 30 minutes, until bubbling and heated through.