1️⃣ In a medium skillet, heat 2 tablespoons of 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 in the last minute or two. 🍳🧅🧄

2️⃣ Spread the onions evenly in the bottom of an oiled 2-quart shallow dish. Let cool. 🥣

3️⃣ Preheat oven to 375°F (190°C). 🔥

4️⃣ Put tomato slices on a shallow plate to drain for a few minutes, and then discard the collected juices. 🍅🫗

5️⃣ In a medium bowl, toss the zucchini and squash slices with 1 1/2 tablespoons of olive oil, 2 tablespoons thyme, and 1/2 teaspoon salt. 🥒🍋🌿

6️⃣ Sprinkle 1 tablespoon thyme over the onions in the dish. 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. 🍅🧀

7️⃣ Next, lay a row of zucchini, overlapping the tomatoes by two-thirds, and sprinkle with cheese. Repeat with a row of squash, then repeat rows, sprinkling each with cheese, until the dish is full. You may want to press the rows tight (like you’re trying to stand them more upright) two or three times as you go along. Season lightly with pepper and remaining 1/2 teaspoon salt. Drizzle remaining 1 1/2 teaspoons olive oil over everything. 🥒🍋🧀🫒

8️⃣ Combine the remaining cheese with the remaining 1 tablespoon of thyme, and sprinkle this liberally over the whole gratin. Bake 65 to 70 minutes, until well-browned all over and juices have bubbled for a while and reduced considerably. 🧀🍲⏳

9️⃣ Let cool at least 15 minutes before serving. 🌬️⏲️

Enjoy your delicious Summer Gratin! 🍅🧀🍆