Heat 1 tablespoon of the olive oil in a skillet over medium heat. Add the onions and cook, stirring occasionally, until soft and golden, 8 to 10 minutes. Let cool slightly.

In a large bowl, combine the rice, sautéed onions, grated tomato, parsley, dill, mint, lemon juice, remaining olive oil and salt. Mix thoroughly.

Place a grape leaf on a clean surface, shiny side down. Spoon about 1 heaping teaspoon of filling onto the base of the leaf. Fold the bottom up over the filling, then fold in the sides and roll tightly to form a cigar shape. Repeat with the remaining leaves and filling.

Line the bottom of a wide, heavy pot with tomato slices. Arrange the grape leaves seam-side down in snug concentric circles. If needed, make a second layer. Sprinkle with za’atar and add the garlic, if using.

Spoon the crushed tomatoes evenly over the rolls. Pour in just enough water to cover. Drizzle with a bit more olive oil and lemon juice. Place a sheet of parchment paper directly over the grape leaves and weigh them down with a heatproof plate. Cover with a lid or foil.

Bring to a gentle boil over medium heat. Reduce the heat to low and simmer for 1 1/2 hours, or until the rice is fully cooked and the leaves are tender. Let cool in the pot.

To make the tzatziki, stir together the yogurt, cucumbers, mint, lemon juice, olive oil, salt and pepper in a bowl. Chill until ready to serve.

Serve the grape leaves warm, at room temperature or chilled, with tzatziki on the side.