3 tbsp olive oil, divided, plus more for greasing the pan
2 large yellow onion, cut in half and sliced
2 clove garlic, minced
1 lb medium round potato, unpeeled
3/4 lb zucchini
1 1/4 lb medium tomatoes
1 tsp kosher salt
1/2 tsp black pepper, freshly ground
1 tbsp fresh thyme leaves, plus extra sprigs
2 oz gruyere cheese, grated
Preheat oven to 375 degrees F (190 degrees C). Brush a 9 x 13 x 2 inch baking dish or 3 quart casserole with olive oil.
In a medium saute pan, heat 2 tablespoons olive oil and cook onions over medium-low heat until translucent, about 8 to 10 minutes. Add garlic and cook for another minute. Spread onion mixture on the bottom of the prepared baking dish.
Slice potatoes, zucchini, and tomatoes in 1/4 inch thick slices. Layer them alternately in the dish on top of the onions, fitting them tightly, making only 1 layer. Sprinkle with salt, pepper, thyme leaves, and thyme sprigs and drizzle with 1 more tablespoon olive oil. Cover dish with aluminum foil and bake until the potatoes are tender, about 35 to 40 minutes. Uncover the dish, remove thyme sprigs, sprinkle cheese on top, and bake for another 30 minutes until browned. Serve warm.