Melt the butter in a large pot, and add the onion and garlic. Saute for 5 minutes until slightly softened. Add the sundried tomatoes, diced tomatoes and thyme, and season with salt and pepper to taste. Bring to a boil, then reduce to a simmer. Simmer it for 20 minutes with the lid on, so the flavors can meld together.
Make any final seasoning adjustments, and add water if necessary, to restore the original consistency of the soup. Puree the soup in a blender or with an immersion blender until smooth (don’t overfill the blender or it may explode because of the heat). Serve with the grilled cheese.
To make the grilled cheese, heat up a skillet over medium heat and coat the bottom of the pan with butter. Lay the slices of french bread in the pan and top with the cheese. Cook for a couple minutes until the cheese melts and the bread is crispy, and golden brown. Press 2 slices together to form sandwiches. Serve with the soup, and enjoy the cheesy tomato goodness!