Using a sharp knife, slice each zucchini in half lengthwise to create 4 'steaks.' Score the flesh side in a crosshatch pattern. Season with salt and let sit for 15 minutes to release moisture.
In a large skillet over medium heat, cook the garlic, red pepper flakes, and 2 tablespoons of olive oil until fragrant, about 1-2 minutes. Transfer to a small bowl.
Heat 1 tablespoon olive oil in same skillet. Pat zucchini dry and cook 2 halves flesh side down until golden brown, about 2-3 minutes. Transfer to baking sheet. Repeat with remaining zucchini and oil.
Brush zucchini with garlic oil. Bake at 425°F until tender, about 8-10 minutes. Top with mozzarella and Parmesan cheeses.
Broil until cheese is melted and browned, about 2-3 minutes. Transfer to platter and top with torn basil and additional red pepper flakes.