1. Heat olive oil in a large pot over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes. Stir in the minced garlic and cook for an additional minute.
2. Add the crushed tomatoes, vegetable broth, and dried basil to the pot. Bring to a boil, then reduce heat and simmer for 20 minutes.
3. Using an immersion blender, puree the soup until smooth. Stir in the heavy cream and season with salt and pepper to taste.
4. While the soup is simmering, prepare the toast. Melt butter in a skillet over medium heat. Add the sliced onions and cook until caramelized, about 15 minutes, stirring occasionally.
5. Preheat the broiler. Place bread slices on a baking sheet. Spoon the caramelized onions onto each slice and top with shredded mozzarella cheese. Broil until the cheese is melted and bubbly.
6. Serve the cheese and caramelized onion toast alongside the creamy tomato soup. Enjoy!