- To prepare the sticky tomatoes, preheat the oven to 325 degrees F. Line a baking sheet with parchment paper.
- Slice the cherry tomatoes in half lengthwise, and the larger tomatoes into slices 1/3 to ½ an inch thick. Place the tomatoes on the baking sheet, and drizzle over the olive oil, gently rubbing the tomatoes around in the oil to coat them on both sides. Sprinkle with the salt and sugar, and place in the oven. Cook for an hour to an hour and 15 minutes, until the tomatoes look a little wrinkled and sticky and, are starting to brown on the bottoms, but still have a little moisture.
- To prepare the frittata, preheat the oven to 325 degrees F. Warm 1 tablespoon of the olive oil in an medium sized, oven-proof sauté pan set over medium heat. When the oil begins to shimmer, add the sliced leek, celery, and ¼ teaspoon of salt. Saute until softened, 5-7 minutes. Set the vegetables aside, and wipe or wash the pan clean.
- Whisk the eggs together in a large bowl. Stir in the sautéed leeks and celery, the grated hard pecorino, the celery and parsley leaves, lemon zest, black pepper, and remaining ¼ teaspoon of salt. Finally, fold in most of the soft pecorino and the sticky tomatoes, reserving a few for the top of the frittata.
- Warm the remaining 2 tablespoons of olive oil in the sauté pan, set over medium-high heat. When the oil begins to shimmer, add the egg mixture. Lay the reserved tomatoes and cheese on top of the egg mixture, and place the pan in the oven.
- Bake for 30-35 minutes, until the top of the frittata is almost set, and the middle is still just slightly wiggly to the touch. Set aside for 10-15 minutes to finish cooking, and serve cut in wedges. The frittata is delicious served warm or cold.