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1
Trim the stems off the mushrooms.
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2
Using a moist paper towel, wipe the caps clean.
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3
Using a sharp object, such as the tip of a metal skewer, poke a series of holes in the gill side of the mushroom caps.
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4
Insert garlic slivers in some holes, cheese slivers in others, rosemary leaves in others, and pine nuts in the remaining holes.
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5
Combine 1/2 cup vinegar and the salt and pepper in a mixing bowl and whisk until the salt is dissolved.
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6
Whisk in the oil and the basil.
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7
Pour some of the mixture in the bottom of a nonreactive baking dish and arrange the portobellos in it, gill side up.
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8
Swish the mushrooms to coat the bottom with marinade.
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9
Spoon the remaining marinade over the mushrooms, cover and refrigerate for as little as 30 minutes or as long as 3 hours.
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10
Preheat grill to high.
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11
When ready to cook, remove the mushroom caps from marinade.
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12
Strain the marinade if the basil looks wilted.
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13
Whisk the remaining 2 tablespoons of vinegar into the marinade.
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14
Arrange the mushrooms on the grill grate, gill side down.
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15
Grill for 3 minutes.
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16
Turn over and spoon the reserved marinade onto the caps.
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17
Continue grilling until browned and tender,(reducing heat if necessary) 4 to 6 minutes, rotating the caps 45 degrees after 2 minutes to create a crosshatch of grill marks.
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18
Remove from grill and serve immediately.