Arrange oven racks in top and bottom thirds of oven. Preheat to 425°F (220°C). Line 2 large baking sheets with parchment paper.
Cut tomatoes in half and place on a baking sheet (if they don’t fit, move some to the second sheet). Drizzle with half of the oil. Sprinkle with rosemary, salt and pepper.
Slice onions thinly. Place onions on another baking sheet; add garlic cloves and thyme sprigs. Drizzle with remaining oil; toss to mix and spread out in a single layer. Place on the bottom rack.
Roast, without stirring, until tomatoes start to burst and onions are softened, about 1 hour. Switch racks halfway through roasting.
Scrape tomatoes (including any pan juices) and onions into a large Dutch oven; squeeze in garlic from papery skins. Using your fingers, crumble in roasted thyme leaves (discard stems). Add broth and basil. Bring to a boil, then reduce heat to medium; partially cover and simmer for 15 minutes.
Working in batches, purée in a blender until smooth. Ladle into bowls and swirl in yogurt, herbs and olive oil, if using. Soup will keep well, covered and refrigerated, for up to five days or portion and freeze for up to three months.
Nutrients per serving: 58 calories, 3 g protein, 3 g fat (1 g saturated fat), 5 g carbohydrates (1 g fibre), 0 mg cholesterol, 46 mg sodium.
Tip: Best made with fresh plum tomatoes straight from the garden or market, it’s a keeper for the off-season, too, when tomatoes have to travel a little farther. Roasting brings out their natural sweetness and concentrates the flavours.
How to customize your recipe
Vegan: Use vegetable broth, or make miso broth: mix 8 cups water with ½ cup organic red miso paste. Miso is salty, so salt to taste the soup at the end of cooking
Keto: Replace olive oil with MCT oil or coconut oil. Coconut oil contains MTC (medium chain triglycerides) which help induce ketosis.
Paleo: Make soup with chicken broth or bone broth. Since the oven is on anyway, roast a few bone-in, skin-on chicken thighs or breasts. Place on the baking sheet with onions. Cool, discard fat and shred meat from bones. Stir into the soup at the end.