Preheat the oven to 425°F.
Cut the heirloom tomatoes into large chunks. Leave the smaller cherry and Sungold tomatoes whole, halving any larger ones.
Add the tomatoes, onion, and garlic to a large sheet pan. Drizzle with oil and season with 1 tsp each of salt, black pepper, and Italian seasoning. Toss until everything is evenly coated.
Roast for 45-60 minutes, or until the tomatoes break down and start to caramelize around the edges.
Transfer everything from the sheet pan, including all of the tomato juices, to a large pot.
Bring to a simmer over medium heat and cook uncovered for 15-30 minutes, letting the tomatoes reduce slightly and the flavors concentrate. The longer you simmer and reduce it, the more concentrated the tomato flavor will become.
Add the chicken stock and honey. Using an immersion blender, blend until completely smooth. You can also carefully transfer the soup to a high-powered blender and blend until smooth.
Reduce the heat to low and stir in the heavy cream and butter until fully incorporated. Taste and season with additional salt and pepper as needed.
Serve warm topped with fresh basil, cracked black pepper, and an extra swirl of heavy cream, if desired.