Wash the tomatoes, quarter the onion, and peel the garlic cloves. Put them in the slow cooker, add in water, pepper, basil and oregano. Let it cook, stirring occasionally.
Add in tomato paste. As the tomatoes become soft, stir to combine. Continue cooking for 4 hours or until it reaches the desired consistency. Use a wooden spoon to roughly break the tomatoes during cooking.
Turn off the heat and let the tomato mixture cool. When cool, puree it in a blender. Pour the pureed mixture back in the slow cooker. Heat it through, then reduce the heat to keep it warm.
For spaghetti meatballs, just add in cooked meatballs to the tomato sauce. Let it simmer on low heat for 10 minutes to let the flavor blend (or you can use this sauce for any pasta you like).