In a large bowl, whisk together the olive oil, lemon juice, minced garlic, and Greek seasoning until well combined.
Add the pressed tofu, red onion, and zucchini to the bowl and toss to coat thoroughly. Cover and marinate for a minimum of one hour and up to overnight in the refrigerator.
Thread the tofu, red onion, and zucchini onto skewers in an alternating pattern. Keep pieces close together without crowding.
Preheat the grill to 400°F. Place the skewers on the grill and cook for 4 to 5 minutes per side until a slight char forms on the tofu. Baste with any remaining marinade after each turn.
STEP SIX: Serve. Transfer the skewers to a serving platter. Spoon or drizzle about a quarter of the tzatziki over the skewers and sprinkle the crumbled feta on top. Serve immediately with the remaining tzatziki on the side for dipping.