In a medium pot, over medium heat, add the canola oil. Stir in the onions and celery, and saute until the onions are soft, about 5 minutes. Add the garlic, red pepper flakes, and salt and pepper, to taste, and saute an additional 2 minutes. Stir in the crushed tomatoes and vegetable broth. Bring to a boil, then reduce the heat to a simmer. Cook, covered for 30 minutes. Add the remaining soup to a blender and puree until smooth. Return the puree to the pot and season with salt and pepper, to taste. Serve hot with grilled cheese sandwich.