Cut the tomatoes and onion into quarters or large pieces. Peel the garlic cloves and leave them whole. Place the vegetables and rosemary on a baking sheet and drizzle with olive oil.
Roast in a 400°F oven for 45 minutes. After roasting, discard the rosemary sprigs.
Place the roasted vegetables in a blender or food processor. Add flour then puree until smooth.
Pour tomato mixture into a pot and add your favorite stock.
Simmer for 10 minutes on a medium heat. Season with salt and pepper to taste.