Heat Oven - Set the oven rack to the center position and preheat to 400ºF (204ºC).
Prepare Tomatoes - Line a large baking sheet with foil. Place sliced Roma and Campari tomatoes cut-side up on the sheet. Evenly disperse the sliced garlic. Drizzle the olive oil over the top. Season with salt and pepper.Flip over some of the tomatoes, so an even amount is cut-side up and skin-side up on the baking sheet. Place the thyme sprigs on top.
Roast - Roast the tomatoes until lightly browned, slightly shriveled, and most of the liquid has evaporated, about 55 to 60 minutes. Discard the thyme sprigs.
Cook Aromatics - Heat a large pot over medium heat. Add butter, once melted, add the minced garlic. Saute until fragrant but not browned, about 30 seconds. Add onions, and saute until lightly browned, 8 to 10 minutes.
Simmer Soup - Add the roasted tomatoes and roasted garlic, bay leaf, vegetable stock, and basil. Bring to a boil, then reduce to a simmer and cook for 20 minutes, occasionally stirring to reduce some of the liquid. Remove bay leaf and discard.
Puree - Use an immersion hand blender to puree the soup until smooth. Alternatively, process the soup in batches in a blender until smooth.
To Serve - Taste and season the soup with salt and pepper as desired. Garnish with chives.
Notes
Recipe Yield: About 5 cups
Serving Size: 1 cup
Adjusting Consistency: If the soup is too thin after blending, continue to simmer until the desired thickness is reached. If too thick, gradually add more vegetable stock.
Make it Creamier: Add a few tablespoons of heavy cream or canned coconut milk. Stir into the soup after blending.
Make it Vegan: Substitute butter for extra-virgin olive oil.
Make it Whole30: Substitute extra-virgin olive oil or ghee for butter. Substitute sea salt for kosher salt.
Storing: Cool and store in an airtight container in the refrigerator for up to 5 days. Freeze in resealable plastic bags for up to 6 months. Defrost before using.
Reheating: Cover and microwave on high setting in 30-second intervals until hot, stirring in between. Alternatively, cook on the stovetop over medium heat until hot.