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1
Preheat the oven to 450F.
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2
In a shallow bowl, combine the balsamic vinegar, about 2 tablespoons of the EVOO, salt, and pepper.
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3
Add the portobello mushrooms and toss to coat.
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4
Arrange the mushrooms on a cookie sheet, gill side up.
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5
Roast the mushrooms for 12 minutes or until tender and cooked through.
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6
While the mushrooms are roasting, prepare the caramelized onion topping.
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7
Heat a medium-size skillet over medium-high heat with the remaining 2 tablespoons of EVOO (twice around the pan).
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8
Add the sliced red onions, garlic, tomato paste, salt, and pepper.
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9
Cook the onions, stirring frequently, for about 8 minutes, or until nice and brown.
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10
While the onions are caramelizing, prepare the ricotta cheese topping for the mushrooms.
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11
In a bowl, thoroughly combine the ricotta cheese and the pesto.
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12
Once the mushrooms have roasted for 12 minutes, remove them from the oven and divide the ricotta mixture evenly among the 4 mushrooms, spreading it onto the gill side of the mushrooms with the back of a spoon.
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13
Return to the oven for about a minute, just to heat the cheese.
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14
Add the chicken stock and chopped olives to the onions.
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15
Turn the heat to high and continue to cook until the stock has reduced almost completely.
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16
Arrange one cheese-topped mushroom on the bottom of each split roll.
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17
Top the cheese with some of the caramelized onions.
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18
Top that with 2 slices of prosciutto and some arugula.
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19
Finish it off with a roll top.
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20
Eat and enjoy!
