Steps
1. Preheat a grill or skillet over medium heat.
2. Brush the portobello mushroom cap with olive oil and season with salt and pepper.
3. Place the mushroom cap on the grill and cook for about 5-6 minutes on each side, until tender.
4. In the last few minutes of cooking, add the fresh spinach on top of the mushroom to wilt it slightly.
5. Toast the slice of whole grain bread on the grill until golden brown.
6. Assemble your burger by placing the grilled mushroom and spinach on the toasted bread. Add cheese if desired.
7. Top with avocado slices and garlic aioli. Serve immediately.