Preheat your oven to 400 degrees F. This initial step is crucial as it gets your oven nice and hot, perfect for roasting. You want to create a beautiful caramelization on the tomatoes and onion.
Line a 13" x 9" rimmed baking sheet with parchment paper. This prevents sticking and makes for an easy cleanup afterward. Plus, it helps the tomatoes roast evenly.
Quarter the tomatoes and thickly slice the onion. This is the time to get your hands a little messy, but that’s part of the fun!
Arrange the tomatoes and onion on the prepared sheet. Make sure they’re spread out evenly; this will ensure they roast beautifully and don’t steam.
Drizzle the olive oil over the vegetables. Don’t be shy; this will help enhance the flavors. Sprinkle salt and black pepper over the tomatoes to season them generously. Toss gently to coat them in the oil and seasonings.
Roast the tomatoes for about 30 minutes, or until you see juice bubbling at the bottom of the pan and the edges of the onion slices are brown. The roasting time may vary slightly based on your oven, so keep an eye on them!
Once done, remove the pan from the oven. Allow the tomatoes and onion to cool slightly, about 5 to 7 minutes. This will make it easier to handle.
Transfer the roasted tomatoes and onion along with all the juices from the bottom of the pan into a blender. This is where the magic happens! Blend until smooth, stopping occasionally to scrape down the sides with a spatula.
The soup will turn thick and creamy, a beautiful texture that’s irresistible. If it seems too thick, you can add a little water or stock for your desired consistency.
Pour the soup into bowls. Garnish with a sprinkle of grated Parmesan and chopped celery leaves if desired. Serve with grilled cheese or croutons, and watch everyone enjoy!