This soup is best when made with ripe, summer tomatoes, roasted to concentrate their natural sweetness. If you’re cheating a bit and using out-of-season tomatoes, you may find that you have to add a bit of sugar to the finished soup, since the tomatillos are also acidic—even though we’re only using a few of them. Or you can substitute canned, diced tomatoes (you’ll need 2½ cups), in which case you don’t need to roast them first. If you’d like, pluck the skins from the fresh tomatoes after they’ve roasted (they’ll peel off easily), but if you have a good blender this step is unnecessary.
2½ pounds ripe, juicy red tomatoes, (such as Oxheart or Delicious), cored and halved; or 2½ cups canned diced tomatoes
4 tomatillos, husks removed, rinsed, and halved
1 medium sweet onion, cut into wedges
1 serrano chile, stemmed and halved lengthwise
¼ cup extra virgin olive oil
1 tablespoon garam masala
2 teaspoons kosher salt, plus more to taste
½ teaspoon freshly ground black pepper, plus more to taste
2 cups vegetable stock
2 fresh thyme sprigs
½ cup heavy cream
Sugar, if needed (see headnote)
Small basil leaves, for garnish
Preheat the oven to 450°F. Spread the tomatoes, tomatillos, onion wedges, and serrano on a baking sheet and drizzle with olive oil. Season with the garam masala, salt, and pepper and toss to coat. Roast for 30 minutes, until the skins of the tomatoes are wrinkled and the onion wedges are soft and beginning to brown.
Transfer the vegetables and any accumulated juices to the blender and blend until smooth, adding some of the vegetable stock as needed to aid the blending (you can do this in batches if necessary). Pour the pureed soup into a large saucepan and stir in the remaining stock and thyme sprigs. Bring to a boil over medium-high heat. Lower the heat to medium and simmer, stirring occasionally, for 10 minutes.
Remove and discard the thyme sprigs. Stir in the cream and season to taste with salt and pepper; if the soup tastes acidic, stir in a bit of sugar. Let simmer for 5 minutes longer, until hot. Ladle into bowls and garnish each bowl with a few small basil leaves. Serve hot.