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1
Preheat oven to 400u00b0F degrees with rack in upper third.
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2
In a medium saucepan, heat 1 tablespoon oil over medium heat; add the onion, garlic, and mushrooms cook, stirring, until soft and lightly golden, about 8 minutes.
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3
Add tomatoes and salt, and cook, stirring occasionally, until sauce has thickened, about 30 minutes.
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4
Stir in vinegar, oregano, basil & Italian seasoning; season with pepper.
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5
Remove sauce from heat.
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6
Meanwhile, heat a large cast-iron skillet or grill pan over medium heat; lightly brush eggplant slices with the remaining tablespoon oil.
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7
Working in batches, lay slices in skillet in a single layer; cook until browned and they begin to soften, 2 to 3 minutes per side.
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8
Transfer to a plate.
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9
Spoon about a cup tomato sauce into a 9-inch square baking dish, spreading to coat evenly; arrange eggplant slices snugly in a single layer.
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10
Spoon about 1 cup tomato sauce over eggplant, and arrange polenta rounds in slightly overlapping slices on top.
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11
Repeat with sauce and another layer of eggplant.
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12
Finish by dotting with remaining tomato sauce.
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13
Cover with foil; bake until bubbling and juicy, about 30 minutes.
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14
Remove foil; continue baking until sauce is lightly caramelized and eggplant is tender, about 15 minutes more.
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15
Remove from oven; let cool slightly, and serve.