Spray a casserole dish with cooking spray and set aside. Melt butter in a medium sized sauté pan and add onion. Cook until onion is softened. Add mushrooms and bell pepper; sauté until mushrooms have given up all their liquid and are softened.
2Stir in flour and cook stirring for 2 minutes (will be very thick). Add chicken stock slowly and whisk until incorporated. Add milk, bring to a boil then reduce heat and simmer for 3 or 4 minutes, stirring constantly.
Add shredded cheddar along with salt, pepper, hot sauce, parsley, pimento and sherry. Stir in cooked meat or shrimp and cooked spaghetti. Transfer to prepared casserole dish.
3Top with grated cheese and sprinkle with panko bread crumbs.
Bake for 25 minutes or until mixture is bubbling and top is golden.