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1
Combine the vinegar, soy sauce, 3 tablespoons olive oil, rosemary, grill seasoning, onion, and garlic in a measuring cup or bowl with a spout.
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2
Allow it to stand while you wipe the dirt off the mushrooms and place them in a large baking dish, gill side down.
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3
Slowly pour marinade over the mushrooms, getting some on each cap.
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4
Let them marinate at room temperature for 30 to 90 minutes, occasionally spooning or brushing more marinade over the mushrooms.
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5
While the mushrooms marinate, wash and halve the baby tomatoes.
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6
If youre really short on time, it wont hurt to leave them whole (theyll just require a bit more cutting to eat).
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7
Just before the mushrooms are done marinating, heat the remaining teaspoon of oil over medium heat in a skillet, then add tomatoes and saute until just softened, no more than 2 minutes.
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8
Add the spinach and cook until wilted, then immediately remove from heat and drain off excess liquid (I drained it in a colander then returned it to the pan).
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9
Sprinkle with salt and pepper.
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10
Heat grill or grill pan to medium.
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11
If using a grill pan, also preheat the oven broiler.
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12
Remove mushrooms from marinade, brushing off seasonings, and place them gill side down on the grill pan for 5 minutes.
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13
They may stick a little but should release easily with a heat-safe rubber spatula.
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14
Turn them over, carefully distribute vegetable filling evenly over the mushroom caps using tongs, and top with cheese.
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15
Grill for another 5 minutes, closing the grill lid to melt the cheese.
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16
If using a grill pan, heat mushrooms under broiler for two minutes (on a baking sheet) to melt and brown the cheese.
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17
Serve mushrooms with more balsamic vinegar and pepper.
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18
Oven-only method: Instead of heating the grill and the oven, heat only the oven to 450 F (not to broil) and position the rack in the top third of the oven.
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19
Line a baking sheet with foil and place the mushrooms gill side down after wiping off the excess marinade.
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20
If you have a wire rack or broiler pan, use that so excess moisture drips off the mushrooms.
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21
Cook for 5 minutes.
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22
Remove from oven, carefully turn to face them with the gills up, then fill with the spinach and tomato mixture and top with cheese.
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23
Return to the oven for another 5 to 8 minutes, or until cheese is melted and beginning to brown.
