Heat oil in a large pot over medium heat. Add onion and garlic and cook for 2-3 minutes, until softened. Add the rest of the vegetables and cook for about 5 minutes, occasionally stirring.
Add the tomato paste and Italian seasoning and sir well. Add pumpkin, tomatoes, balsamic vinegar and bring pot to a boil. Reduce the heat to low and simmer, covered, for about 30 minutes.
Remove from heat. Cool slightly and transfer the sauce into a blender. You can also use an immersion blender. Puree until smooth. If you feel the sauce is too thick, add water or vegetable broth until desired consistency is reached.
