Preheat the oven to 400°F. Position a rack in the center of the oven. Line a large rimmed baking sheet with parchment paper for easy cleanup and to prevent sticking.
Rinse and dry the Roma tomatoes. Slice each one in half lengthwise and remove the stem/core if present. If you prefer a slightly less acidic soup, you can gently scoop out some of the seeds, but it’s not required.
Peel the yellow onion and cut it into quarters so the pieces roast evenly. Peel the garlic cloves and leave them whole so they soften and sweeten in the oven without burning.
Spread the tomatoes (cut-side up), onion quarters, and garlic cloves on the prepared baking sheet, leaving a little space between pieces for good airflow. Drizzle the olive oil evenly over everything.
Season generously with salt and black pepper. Toss lightly with your hands, then flip the tomatoes back to cut-side up so they caramelize nicely on top.
Roast for 35–45 minutes, rotating the pan halfway through. The tomatoes should look collapsed with lightly blistered edges, the onion should be tender with some browning, and the garlic should be soft and golden.
If anything is browning too quickly near the edges, move those pieces toward the center of the pan.
Remove the baking sheet from the oven and let the vegetables cool for about 10 minutes.
Tilt the tray and use a spatula to gather every bit of those flavorful juices and any caramelized bits - this is liquid gold for your soup.
Transfer the roasted tomatoes, onion, garlic, and all pan juices to a blender. Add the tomato paste. If your blender is small, work in batches.
For hot blending, vent the blender lid (remove the center cap) and cover the opening with a clean kitchen towel to allow steam to escape safely. Blend on high until completely smooth and velvety.
If the soup seems thicker than you like, blend in a splash of hot water, a little at a time, until it reaches your preferred texture.
Taste and add a pinch more salt and pepper if needed.
Pour the blended soup into a pot and set it over low to medium-low heat for 5–10 minutes, stirring occasionally. This brief warm-up helps the flavors round out without overcooking the fresh notes.
Stack the basil leaves, roll them up, and slice into thin ribbons (chiffonade).
Ladle into bowls, garnish with the basil, and add a tiny grind of black pepper or a light drizzle of olive oil, if you like. Enjoy!