Wash, core, and cut tomatoes into medium-sized chunks. Blend in a food processor or blender until a consistent paste forms. You may need to do this in several batches depending on the size of your machine.
Heat olive oil in a large stock pot, and add garlic and onions. Sautee until softened.
Add the tomato puree and bring it to a boil. Add the Italian seasoning, sugar, and pepper. Depending on the acidity of your tomatoes, you may need more or less sugar.
Let the sauce simmer until reduced and thickened to your desired consistency. Depending on the moisture level in the tomatoes you used, the sauce might need to simmer anywhere from 1 hour to 2 hours.
Store in the refrigerator for up to two weeks, or in the freezer for long-term storage.
*Note: this recipe has not been tested for canning purposes. Please seek a canning recipe for any water bath canning needs.