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1
Preheat oven to 375.
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2
To cook the onions-- In a medium skillet, heat the olive oil over medium heat.
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3
Add the onions and saute, stirring frequently, until limp and golden brown, about 2 min.
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4
Reduce the heat to medium-low if they're browning too quickly.
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5
Add the garlic and saute until soft and fragrant, 1 to 2 min.
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6
Spread the onions and garlic evenly in the bottom of an oiled 2-qt. shallow gratin dish (preferably oval).
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7
Let cool.
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8
To assemble the gratin: Put the tomato slices on a shallow plate to drain for a few minutes and then discard the collected juices.
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9
In a medium bowl, toss the zucchini and squash slices with 1-1/2 Tbs. of the olive oil, 2 Tbs.of the thyme, and 1/2 tsp.of the salt.
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10
Reserve half of the cheese for the top of the gratin.
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11
Sprinkle 1 Tbs.of the thyme over the onions in the gratin.
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12
Starting at one end of the baking dish, lay a row of slightly overlapping tomato slices across the width of the dish and sprinkle with a little of the cheese.
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13
Next, lay a row of zucchini, overlapping the tomatoes by two-thirds, and sprinkle with cheese.
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14
Repeat with a row of squash, and then repeat rows, sprinkling each with cheese, until the gratin is full.
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15
Season lightly with pepper and the remaining 1/2 tsp.salt.
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16
Drizzle the remaining 1-1/2 Tbs.olive oil over all.
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17
Combine the reserved cheese with the remaining 1 Tbs. thyme and sprinkle this over the whole gratin.
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18
Cook until well-browned all over and the juices have bubbled for a while and reduced considerably, 65 to 70 min.
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19
Let cool for at least 15 min.before serving.
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20
NOTE: You can use any combination of summer squash for this dish.
