In a microwave safe casserole dish place the onion and butter and cook on full power for 1 1/2 minutes.
Blend in flour, seasonings, water, tomatoes and bouillon.
Cook on full power for 4 minutes or until thickened.
Add potatoes.
Cook uncovered for 13-15 minutes on full power.
Add grated Parmesan cheese to the top of the casserole.