Heat the oven to 350°F/180°C convection.
Blanch the Brussels sprouts in boiling water until bright green and just tender, about 3 minutes. Halve large sprouts, then transfer to ice water, drain, and dry.
Toss Brussels sprouts with olive oil, salt, and pepper and spread on a parchment-lined baking sheet. Quarter tomatoes and arrange with garlic and chili on a second sheet; drizzle with olive oil and season.
Roast both trays until sprouts are browned and tomatoes are collapsed and caramelized, 25–30 minutes, rotating trays once. Blend roasted tomatoes until smooth and strain; chill.
Toss cilantro, parsley, za’atar, lemon juice, olive oil, salt, and pepper with the cooled sprouts, radish, and red onion.
Spoon tomato gazpacho onto plates, top with the salad, and finish with a drizzle of olive oil and a pinch of salt. Serve cool or at room temperature.