Portabella Burger

1. Heat 1½ tbsp oil in a large nonstick skillet over medium-high heat. Add sliced onions; cook, stirring occasionally, until onions are softened and translucent, about 7 minutes. Add balsamic vinegar; cook, stirring for 1 minute. Remove from heat and set aside.
2. In a small bowl, whisk together mayonnaise and pesto; set aside.
3. Place portabella mushrooms on a plate, brush tops and gills with remaining oil. Transfer to pre-heated grill, gill side down; cook 5-6 minutes. Flip and cook another 5-6 minutes.
4. Top each portabella slice with cheese; allow cheese to melt, about 1-2 minutes. Remove from heat.
5. To assemble, spread the bottom of each bun with pesto mayo. Top with grilled portabella, sautéed onions, and fresh arugula.

Tips:

• Skip the grill. The portabella mushrooms can also be baked in the oven. Place the mushrooms gill side up on a baking tray and cook in a 425°F oven for 15-20 minutes.
• Mix up the cheese! Try sliced cheddar, pepperjack, or Swiss. Crumbled cheeses like goat or blue cheese also work well.
• Toasting the buns adds a nice crispness to this burger.