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1
Whisk together the vinegar, salt, pepper, garlic and olive oil.
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2
Toss with the mushroom caps in a wide bowl.
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3
Rub the marinade over the tops of the mushroom caps and place them on a baking sheet, rounded side up.
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4
Let sit for 15 minutes.
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5
Dont rinse the bowl, because youll use any oil and vinegar residue to dress the spinach.
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6
Bring a large pot of water to a boil, salt generously and blanch the spinach for 20 seconds.
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7
Transfer to a bowl of ice water, drain and squeeze dry.
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8
Chop coarsely and toss in the bowl with the residue of the marinade.
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9
Set aside.
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10
Prepare a medium-hot grill, heat a heavy skillet over medium-high heat, or preheat an electric grill or panini pan on medium-high.
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11
Season the mushrooms as desired with salt and pepper.
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12
Place the mushrooms on the hot grill or pan, rounded side down.
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13
Cook them for about 6 to 8 minutes, depending on the thickness, until lightly browned, and moist.
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14
Turn over and cook for another 6 minutes.
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15
Turn over for a minute to reheat the top, then flip back over and place the cheese on top.
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16
Continue to cook until the cheese melts.
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17
If using a panini grill, cook 6 to 8 minutes.
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18
Open the grill and place the cheese on top.
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19
Wait for it to melt.
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20
Place a mound of spinach on the bottom half of each hamburger bun and place the mushrooms, rounded side up, on top of the spinach.
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21
Top with your choice of condiments and serve.
