Large portobello mushrooms are perfect burger material, just the right size for a meaty and satisfying meal. I like them best with Gruyère cheese on top.
03Prepare a medium-hot grill, heat a heavy skillet over medium-high heat, or preheat an electric grill or panini pan on medium-high. Season the mushrooms as desired with salt and pepper. Place the mushrooms on the hot grill or pan, rounded side down. Cook them for about 6 to 8 minutes, depending on the thickness, until lightly browned, and moist. Turn over and cook for another 6 minutes. Turn over for a minute to reheat the top, then flip back over and place the cheese on top. Continue to cook until the cheese melts. If using a panini grill, cook 6 to 8 minutes. Open the grill and place the cheese on top. Wait for it to melt. Place a mound of spinach on the bottom half of each hamburger bun and place the mushrooms, rounded side up, on top of the spinach. Top with your choice of condiments and serve.