Ricotta Smothered Mushroom Burgers with Sweet Onion, Prosciutto and Arugula

When I went to the grocery store the other day they had portobello mushrooms on the 50% off rack.  I decided that I could use those and decided to purchase them.  When I want to make supper tonight I again consulted Rachel Rays 365: No Repeats a Year of Deliciously Different Dinners and spotted recipe #44, which also used arugula and proscuitto, two other ingredients I happened to have in my fridge at the moment.  So I decided to use the Rachel Ray recipe as a starting point to make the following supper.  Enjoy.

Ingredients
4 TBSP balsamic vinegar
4 TBSP Extra Virgin Olive Oil
4 Large Portebello mushroom caps
2 Large Sweet Onions
2 cloves of garlic, minced
11/2 cups ricotta cheese
4 rolls
8 slices of prociutto
1 small bunch of arugula

Warm a medium size frying pan and add the 2 TBSP of the olive oil.  Add the two onions thinly sliced, 2 TBSP of the balsamic vinegar and sald and pepper.  Cook over medium low heat for about twenty minutes.
Combine the last 2 TBSP of vinegar and olive oil in a small bowl.  Brush over both sides of the mushroom.  Grill the mushrooms over medium high heat for about four minutes a side.  Once grilled remove from BBQ, fill the mushrooms some of the ricotta cheese ( I also added a little oregano on the top of mine), then place back on the grill for about five minutes.  Toast the buns on the grill.  When done assemble the burger.  Place the bun, 2 slices of prociutto, then the mushroom, arugula, caramelized onions and two more slices of prociutto.