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1
Place the vinegars, garlic, measured salt, and measured pepper in a medium, nonreactive bowl.
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2
While whisking continuously, add the oil in a slow, steady stream until evenly incorporated.
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3
Place the mushrooms cap-side down in a single layer on a baking sheet and pour the marinade over them.
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4
Turn several times to coat, and let sit at room temperature for 30 minutes.
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5
Heat a grill pan or outdoor grill to high (about 450 degrees F to 550 degrees F).
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6
Slice the ciabatta widthwise into 4 pieces, each slightly smaller than the width of the mushrooms (you may have some bread leftover).
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7
Slice each piece in half horizontally to make 4 tops and 4 bottoms.
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8
Place the halves cut-side down on the grill and cook until brown and toasted, about 2 to 3 minutes.
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9
Remove the bread to a cutting board or work surface.
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10
Spread 1 heaping teaspoon of the pesto on each ciabatta top and bottom (2 teaspoons per sandwichyoull probably have some pesto leftover).
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11
Place 2 tomato slices on each bottom and season with salt and pepper.
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12
Set aside while you grill the mushrooms.
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13
Place the mushrooms cap-side up on the grill, close the grill, and cook until just starting to become tender, about 4 minutes.
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14
Flip the mushrooms, top each with a quarter of the cheese, close the grill, and continue cooking until the mushrooms just give way when pierced with a knife and the cheese is melted, about 4 minutes more.
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15
Remove the mushrooms from the grill.
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16
Stack 2 mushrooms on top of each other, cheese side up, and place on a bottom piece of bread.
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17
Repeat with the remaining mushrooms and bread bottoms.
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18
Top with the lettuce leaves and bread tops and serve immediately.
