Slice spaghetti squash into 1" thick rings. Scrape out seeds and bake 35-40 minutes or until tender. Scrape with a fork to create strands.
Cook onion, garlic and bell pepper in butter until tender. Add cream cheese and Italian seasoning, then cook until melted. Stir in broth and cream slowly until smooth. Simmer until thick and bubbly.
Remove from heat, add parmesan cheese and 1 cup cheddar. Mix until smooth and season with season and pepper to taste.
Combine spaghetti squash, chicken, cream sauce and canned tomatoes, mix well. Spread in a greased 3qt baking dish.
Top with remaining cheddar cheese and bake 25-30 minutes or until hot and bubbly.