For the Slow Cooker Tomato Soup: Place tomatoes into a 6-quart slow cooker. Stir in the butter, tomato paste, broth, potato, garlic, onion, basil, red pepper flakes, sugar, and salt and pepper to taste, about 1/2 teaspoon each. Stir until combined.
Cover and cook on HIGH for 3-4 hours or on LOW for 7-8 hours. Puree with an immersion blender (or regular blender in batches) until desired consistency is reached. Stir in heavy cream; season with salt and pepper, to taste. Garnish with additional red pepper flakes, if desired, and Parmesan Croutons.
For the Parmesan Croutons: Preheat the oven to 375 degrees. Lightly oil a baking sheet with olive oil or nonstick spray. In a bowl, add bread cubes; drizzle with olive oil and toss to coat. Spread in an even layer on the baking sheet; sprinkle with Parmesan cheese. Bake until crisp and golden, about 10-12 minutes, stirring halfway through. Let cool.
Notes
If using whole tomatoes, mash with the back of a spoon before cooking.