Preheat oven to 400 degrees F. Line a 13" x 9" rimmed baking sheet with parchment paper.
Quarter tomatoes, thick slice the onion. Arrange all on prepared sheet.
Drizzle with olive oil. Sprinkle salt and pepper over tomatoes. Toss gently to coat.
Roast tomatoes for about 30 minutes OR until there is juice on the bottom of pan and onion slices are brown at the edges. Do not over bake or they will dry out.
Remove pan from oven and let cool slightly, about 5 to 7 minutes.
Place tomatoes, onion along with all of the juices from the bottom of the pan into a blender. Blend until smooth. Stop and scrape the sides of blender with spatula if needed.
The soup will be thick and creamy.
Pour into bowl, garnish and serve with grilled cheese or croutons.
Notes
I used on-the-vine variety of tomatoes. Any kind will work. You can use carrots and butternut squash in place of tomatoes. The roasting time may be longer. You can add chicken or vegetable stock or broth to the soup to make a larger amount.
Please note, that the nutrition value can vary depending on what product you use. The information below is an estimate.