Grilled Mushroom "Burger"

I've made pizza by substituting a large Portabello Mushroom for the dough, and that is delicious but ends up tasting more like pizza toppings than pizza with a crust.  The Portabello Pizza also is quite a bit softer and requires a fork & knife.  I found this recipe this morning while looking for Meatless Monday Ideas, and I'm sharing before I try it.  I'll give a review when I do; I'm not sure that I will make the bean spread.  If you try this as is, let me know what you think.

"Portobello mushrooms are a nutritious and satisfying alternative to burgers made with beef or turkey. Here they are accompanied by a spread of pureed cannellini beans for added protein -- and flavor. To clean the mushroom caps, wipe them with a damp paper towel; do not rinse or soak, or they will remain soggy even after cooking."

- Whole Living

Ingredients:
2 large garlic cloves, minced
2 tablespoons chopped fresh thyme
1 cup canned cannellini beans, drained and rinsed
2 tablespoons plus 2 teaspoons extra-virgin olive oil
Freshly ground pepper
8 large portobello mushroom caps (about 1 pound), cleaned
1 tablespoon plus 1 teaspoon balsamic vinegar
2 large red onions (about 1 pound), sliced 1/4 inch thick
4 whole-grain hamburger buns
4 thin slices cheddar cheese (2 ounces)
1/2 small bunch arugula (about 2 1/2 ounces), washed and dried


Directions:

1. Heat a grill or grill pan to medium (if using a charcoal grill, coals are ready when you can hold your hand 5 inches above grill for just 6 to 8 seconds). In a food processor, puree one-quarter of the garlic (about 1/2 teaspoon), 1 tablespoon thyme, the cannellini beans, and 1 teaspoon oil until smooth. If mixture is too thick to spread, add a bit of water. Season with pepper, and pulse to combine.
2. Combine the remaining garlic and remaining 1 tablespoon thyme with 1 tablespoon plus 2 teaspoons oil in a shallow dish. Place the mushroom caps in the garlic mixture, and turn to coat. Season with pepper; drizzle with vinegar.
3. Place onion slices on a plate, and lightly coat each side with remaining 2 teaspoons oil. Grill the onions until lightly charred on the first side, about 3 minutes. Flip the onions, and continue grilling until tender and charred on the other side, about 3 minutes more. Transfer onions to a clean plate; cover to keep warm.
4. Working in batches if necessary, place mushrooms on the grill, stem side up. Grill until browned on the first side and juices have begun to collect in the centers, about 5 minutes. Flip mushrooms, and continue cooking until stem side of each cap is browned and center is tender, about 4 minutes more.
5. Split hamburger buns, and place cut side down on grill; cook just until warm and toasted. Spread 1/4 cup bean puree on the bottom half of each bun, and top with 2 grilled mushroom caps. Layer each with sliced cheese, grilled onions, and a small handful of arugula. Top with remaining roll halves, and serve.