Preheat the oven to 400 degrees F. Slice the tomatoes in half lengthwise and place on a large sheet pan. Cut the onions in half and then each half into 4 equal sections. Place on the sheet pan.
Peel and discard the outer layers of the entire garlic bulb making sure to leave the skins of the individual cloves intact (and keep the bulb together as much as possible) Using a sharp knife, cut 1/4th an inch from the top of the cloves.
Wrap the entire bulb tightly with tin foil, adding 2 teaspoons olive oil before sealing it completely. Place the wrapped garlic on the sheet pan along with the tomatoes and onions.
Pour 1/4 cup olive oil over the tomatoes and onions. Add about 1 teaspoon salt and 1/2 teaspoon freshly cracked pepper and toss evenly. Spread into one even layer and roast for 40-45 minutes.
In an 8-quart stockpot over medium heat, melt the butter. Once melted, add the canned tomatoes, basil leaves, fresh thyme, chicken stock, and red pepper flakes. Add the oven-roasted tomatoes and onions* (Read the note if you want a chunkier tomato basil soup.) including the liquid on the baking sheet. Coarsely chop 6-7 of the roasted garlic cloves and add them in. Add another 2 teaspoons salt (or to taste) and an additional teaspoon of cracked pepper.
Bring the mixture to a boil and simmer uncovered for 40 minutes. Taste for any additional seasonings.
When finished, transfer the mixture (in 2-3 batches) to a high powered blender and blend until smooth.
*If you want a chunkier tomato basil, use a food mill or immersion blender. If you do this, you’ll want to coarsely chop the onions after being roasted and before adding them to the stockpot.