How to Make It
Preheat the oven to 425°. Toss the zucchini with 1 tablespoon of the olive oil and spread in a large glass or ceramic baking dish in an even layer. Sprinkle the garlic and dill over the zucchini and season with salt, if using fresh zucchini, and with pepper. Scatter the feta on top and cover with the tomatoes. Season with salt, if using fresh tomatoes, and with pepper and drizzle the remaining 2 tablespoons of olive oil all over.
Bake the gratin for 40 minutes if using frozen vegetables or for 55 minutes if using fresh, or until all the liquid in the dish has evaporated and the zucchini is sizzling. Serve the gratin warm or at room temperature.