Roasted Portobello Mushroom Cheeseburgers with Caramelized Onions and Pimento Aioli

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For the burgers:

Juice of 1 lemon (2 to 3 tablespoons)

1/4 cup extra virgin olive oil

2 tablespoons worcestershire sauce

2 teaspoons fresh thyme leaves

1/4 teaspoon garlic powder

6 portobello mushrooms, wiped clean and stems removed 

6 portobello mushrooms, wiped clean and stems removed

Kosher salt and freshly ground black pepper

6 slices Muenster cheese (or Swiss, white cheddar, provolone-your choice)

6 Portuguese muffins or brioche hamburger buns, buttered and toasted

1 1/2 cups loosely packed arugula

For the onions:

1 tablespoon olive oil

1 medium Vidalia onion, thinly sliced

1/4 cup water

Kosher salt and freshly ground pepper

For the aioli:

1/2 cup mayonnaise

2 tablespoons jarred chopped pimentos, drained

1 tablespoon red wine vinegar

1 garlic clove, minced

1 tablespoon minced flat-leaf parsley

Kosher salt and freshly ground black pepper