1. Preheat oven to 450° with racks in upper and lower thirds. Line 2 large (4-sided) baking pans with foil.
2. Arrange cauliflower on baking pans and drizzle with 2 tablespoons oil, salt, and 1/4 teaspoon pepper. Roast, switching position of pans after 15 minutes, until golden in spots and just tender (30 minutes total).
3. While cauliflower roasts, mince and mash garlic to a paste and place in a small bowl; whisk in remaining 2 tablespoons oil, olives, lemon zest and juice, and remaining 1/4 teaspoon pepper. Arrange cauliflower on a platter and drizzle with vinaigrette.