Heat a grill to medium-excessive. Well-organized and evenly oil sizzling grill. Season steaks with salt and pepper; grill, 3 to 4 minutes per side for medium-rare, flipping once. Tent with foil on a cutting board. Let leisure 10 minutes. Meanwhile, brush zucchini and crimson onion with olive oil; season with salt and pepper. Grill vegetables till crisp-soft, 2 to a diminutive while per side, flipping once. In a small bowl, mix mayonnaise and garlic.