Preheat oven to 350. Cook pasta and set aside. Cook chicken, dice and set aside. In a blender puree heavy cream, sour cream, garlic powder, chicken base and peppers.
In a skillet melt butter, add onion and green pepper and cook until tender. Add minced garlic and cook for a few more minutes. Now add chicken and sauce let simmer for about 5 mins. Season with salt if needed. (you can thicken sauce with a slurry of cornstarch and water, if desired.)
Add chicken and sauce to cooked pasta, mix well. Pour into a casserole dish and top with cheese. Bake until cheese is melted, about 20 mins. Remove from oven, garnish with diced tomatoes and scallions.