Preheat George Foreman grill by plugging it in, or setting it to MAX, 400°F or setting 4. In a small bowl, stir together onion, garlic, poppy seeds, sesame seeds and sea salt. Set aside.
Remove leaves from cauliflower and cut out core. Slice cauliflower into 1” thick slices.
Note: Depending on how large cauliflower is, it may yield only 3 steaks, or up to 5 for a larger head of cauliflower.
Brush both sides of cauliflower steaks with oil.
Place on preheated grill and sprinkle half of seasoning over steaks. Close lid and grill for 8 minutes. Flip steaks and sprinkle with remaining seasoning.
Close lid and grill for additional 8 minutes, until tender.