Preheat oven @ 300 degrees. Cook the spaghetti according to the directions on the box. Chop and saute onion in melted butter until golden. Drain the spaghetti and onion. Place in a dish sprayed with Pam. Add cubed cheese and tomatoes. Mix all ingredients. Cook for 3 hours @ 300 degrees. Check after 2 hours. My family likes the top crispy, so we leave it in the oven longer. Make sure you leave it in the oven long enough for all the cheese to melt. Also, I usually triple the recipe because everyone in my family loves it, so it does take 3 hours the more spaghetti and cheese you use. If you are a cheese lover like we are, I add at least a half a pound more cheese than what it calls for.