Summer Zucchini & Squash Gratin Provencal

Since this would be the 100th birthday of Julia Child, I am cooking French style this week!  Also since my fridge is loaded with fresh summer squash and zucchini and I have loads of garden fresh tomatoes, I made this wonderfully yummy recipe, which I adapted slightly from Fine Cooking.   I added a Panko crumb topping which gave it a nice "crunch" and accompanied the vegetables nicely. 

For the onions:
2 Tbs. olive oil
2 medium onions (14 oz. total), thinly sliced
2 cloves garlic, minced

To assemble the gratin:
1-1/4 lb. ripe red tomatoes, cored and cut into 1/4-inch slices
3/4 lb. (about 2 small) zucchini or other green summer squash, cut into 1/4-inch slices on the bias
3/4 lb. (about 2 small) yellow summer squash or golden zucchini, cut into 1/4-inch slices on the bias
3 Tbs. olive oil
1/4 cup fresh thyme leaves
1 tsp. coarse salt
1-1/4 cups freshly grated parmigiano reggiano
Freshly ground black pepper to taste

Panko bread crumbs (about ½ cup to sprinkle on top)

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To cook the onions: In a medium skillet, heat the olive oil over medium heat. Add the onions and sauté, stirring frequently, until limp and golden brown, about 20 minutes. Reduce the heat to medium-low if they're browning too quickly. Add the garlic and sauté until soft and fragrant, 1 to 2 minutes Spread the onions and garlic evenly in the bottom of an oiled 2-qt. shallow gratin dish (preferably oval). Let cool.

To assemble the gratin: Heat the oven to 375°F. Put the tomato slices on a shallow plate to drain for a few minutes and then discard the collected juices. 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. Reserve half of the cheese for the top of the gratin. Sprinkle 1 Tbs. of the thyme over the onions in the gratin. 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. Next, lay a row of zucchini, overlapping the tomatoes by two-thirds, and sprinkle with cheese. Repeat with a row of squash, and then repeat rows, sprinkling each with cheese, until the gratin is full.

Season lightly with pepper and the remaining 1/2 tsp. salt. Drizzle the remaining 1-1/2 Tbs. olive oil over all. Combine the reserved cheese with the remaining 1 Tbs. thyme and sprinkle this over the whole gratin. Sprinkle Panko bread crumbs over top.  Cook until well-browned all over and the juices have bubbled for a while and reduced considerably, 65 to 70 minutes. Let cool for at least 15 min. before serving.