Preheat the oven to 425°F (220°C).
Place the tomatoes and onions on a baking sheet. Cut the garlic bulbs in half horizontally, cut off the pointy end of the bulbs, and place one part of the garlic head on top of the other. Pour olive oil over the vegetables, toss to coat, and sprinkle with salt. Bake for 45 minutes until the tomatoes are browned and very soft. Set aside to cool.
6 lbs (2.7 kg) medium or large tomatoes, 3 medium onions, 2 heads garlic, 1/4 cup (60ml) olive oil
When cool enough to handle, discard the skins of the tomatoes (although if you have a high-speed blender, it will be able to process them until smooth, so you don't have to do this). Transfer the peeled tomatoes to a large pot, squeezing them slightly with kitchen tongs over the baking sheet to release the tomato juices. Peel the onions, discard the skin, and transfer them to the pot. Squeeze out all the garlic to the pot and discard the skins.
Transfer the tomato juices from the baking sheet to a small or medium pot, along with any additional juices that have gathered in the pot with the tomatoes. Add the tomato paste and chicken broth. Cook over medium-high heat for 20 minutes to thicken the juices and boost the flavor from the tomatoes and broth. The liquid should reduce in volume by at least one-third.
2 tablespoons tomato paste, 2 cups (480ml) chicken broth
Add the reduced liquid to the tomatoes in the pot. Add the basil leaves and red pepper flakes. Blend until smooth.
a couple of basil leaves, small pinch of red pepper flakes
Season with salt and pepper to taste.
Enjoy!