Combine the canned tomatoes, water, salt, onion powder and garlic powder in a medium saucepan. Bring to a boil over medium-high heat and then simmer for 2 minutes. Remove from the heat and puree with an immersion blender until smooth (or transfer to a traditional blender and blend, then return blended soup to the pan.) Return to the stove and add the butter and mascarpone cheese to the soup. Stir over low heat until melted and creamy - about 2 minutes. Remove from the heat and stir in the sweetener, apple cider vinegar, dried basil, and pesto. Serve warm. Store any leftovers in a covered container in the refrigerator for up to 5 days, or in the freezer for up to three months.