Cauliflower Steak Sandwiches

Preparation

1. Preheat the oven to 400˚ F. Place parchment paper on a roasting pan.
2. Trim the leaves off the cauliflower and cut off the bottom of the stem.Cut the cauliflower head in half. Cut each half into 1” to 3/4” slices, leaving the core in place. Cut off the smaller ends of the cauliflower and save for another recipe. There should be 4 cauliflower steaks.
3. Mix olive oil, garlic powder, onion powder, salt and pepper.
4. Lay the cauliflower on the parchment lined baking sheet.Using half of the olive oil mixture, brush onto the steaks.  Place in the preheated oven for 20 minutes.  Remove from the oven and flip the steaks over.  Brush steaks with remaining olive oil and roast for about 20 more minutes, until they are golden brown on the edges. 
5. Slice the red onion. Place each steak on half of the hamburger buns, spread with 2 teaspoons mayonnaise, 1 teaspoon mustard, a few red onion slices, lettuce leave and the other half of the hamburger bun.