To make a veggie soup with basil sauce, you'll first need to prepare the ingredients for the soup. Peel and slice the onion, garlic, carrots, and celery. Add the vegetables to the pot and cook over medium-high heat until they're tender, about 30 minutes. Stir in the tomato paste and salt and pepper to taste. Bring the mixture to boil, then reduce the heat to medium and simmer until it's reduced by half, 10 to 15 minutes, or until the veggies are tender and the sauce is thickened. Serve with sour cream and Parmesan cheese.