To make the soup: Heat the butter in a large saucepan over medium heat. Add the onions and garlic and cook until soft, about 4 minutes,
stirring often.
Whisk in the broth, softened cream cheese, tomatoes (including juices), tomato sauce, basil, stevia, and pepper. Simmer for 5 minutes, stirring occasionally; do not allow it to come to a boil.
Remove the pan from the heat and puree the soup with an immersion blender until smooth. Return the pan to low heat and cover to keep warm while you make the grilled cheese.
Preheat a skillet, grill pan, CHAFFLE WAFFLE MAKER or panini maker to medium-high heat. Slice the chunk of cheese into ¼-inch-thick slices. Brush each side with the oil. Place in the hot skillet, grill pan, or panini maker and cook for 3 minutes per side, or until golden brown (and with grill marks if using a grill pan or panini maker).
Remove the cheese slices from the pan or panini maker and serve with the tomato soup. Garnish each bowl of soup with a tablespoon of sour cream and a few basil leaves, if desired.