Preheat your oven to 375°F (190°C). Line a large baking sheet with aluminum foil for easy cleanup. Cut the Roma tomatoes lengthwise (from stem to bottom). Place them cut-side up on the baking sheet, drizzle generously with olive oil, and season with half of the salt and pepper. Roast in the preheated oven for 1 hour until the tomatoes are soft and slightly caramelized around the edges.

While the tomatoes are roasting, prepare your vegetables. Dice the onion into small, uniform pieces (about ¼-inch). Peel and mince the garlic cloves finely. Remove thyme leaves from stems and mince.

In a large Dutch oven or heavy-bottomed pot, melt the butter over medium-high heat. Once the butter is melted and bubbling, add the diced onion. Sauté for about 5 minutes, stirring occasionally with a wooden spoon, until the onion becomes soft and translucent but not browned. Add the minced garlic and thyme, and cook for another minute until fragrant, stirring constantly to prevent the garlic from burning.

Add the can of crushed tomatoes (including all liquid), chopped basil, and sugar to the pot. Reduce heat to medium-low and let simmer for 10 minutes, stirring occasionally to prevent sticking to the bottom.

Once the roasted tomatoes are done, carefully add them to the pot along with any juices from the baking sheet. Pour in the chicken stock and remaining salt and pepper. Bring to a gentle simmer and cook for 30 minutes uncovered, stirring occasionally. The soup should slightly reduce and thicken during this time.

Turn off the heat. If using an immersion blender, blend the soup directly in the pot until completely smooth, about 2-3 minutes. If using a regular blender, let the soup cool slightly, then carefully transfer it in batches to the blender. Blend each batch until smooth, then return to the pot. Be careful when blending hot liquids as they can expand.

Stir in the heavy cream and taste for seasoning. Add more salt, pepper, or sugar if needed. If the soup is too thick, add a bit more chicken stock to reach desired consistency. If too thin, simmer for a few more minutes to reduce.