Preheat your oven to 400°F (200°C), and toss the tomatoes with a tablespoon of olive oil. Spread them on a baking sheet and roast for about 30 minutes until slightly charred and fragrant.
Meanwhile, heat a tablespoon of olive oil in a pan over medium heat. Sauté the chopped onion until soft and translucent, about 5 minutes, and then add the minced garlic for another minute until fragrant.
Place the roasted tomatoes into your slow cooker, along with the sautéed onions and garlic. Pour in the vegetable broth, and sprinkle the dried thyme, salt, and pepper to taste.
Cover the slow cooker and set it to low. Let the soup cook for about 4 hours, allowing all the flavors to meld and deepen.
After cooking, use an immersion blender or transfer the soup carefully to a blender to puree until smooth and creamy. Be cautious of hot splashes!
Return the blended soup to the slow cooker or a pot, taste and adjust seasoning with salt and pepper as needed. Warm it through if necessary.
Chop fresh basil and stir it into the soup just before serving for a bright, herbal finish.
Serve the hot, velvety tomato soup in bowls, garnished with additional basil if desired. Enjoy the rich, smoky flavor with crusty bread.
