Step 1: Preheat your oven to 375°F (190°C). Brush a 9x13-inch baking dish or a 10-inch oven-safe skillet with 1 tablespoon of extra virgin olive oil.
Step 2: Heat the remaining tablespoon of olive oil in a skillet over medium heat. Sauté the diced red onion for 3-4 minutes until softened and translucent.
Step 3: Add the chopped baby spinach to the skillet. Cook for 1-2 minutes just until the leaves are fully wilted and any liquid released has completely evaporated. Remove from heat and let cool slightly.
Step 4: In a large bowl, vigorously whisk the 10 eggs, milk, dried oregano, garlic powder, sea salt, and black pepper together until the mixture is uniform and slightly frothy.
Step 5: Stir the cooled sautéed onions, spinach, and crumbled feta cheese into the egg mixture.
Step 6: Pour the egg and vegetable mixture into the prepared baking dish, using a spatula to ensure the spinach and cheese are evenly distributed.
Step 7: Arrange the thin zucchini ribbons and red bell pepper strips on top of the egg mixture, weaving them over and under each other to create a beautiful crisscross lattice pattern. Nest the halved cherry tomatoes into the open spaces of the lattice.
Step 8: Place the dish on the center rack of the oven and bake for 25-30 minutes. The frittata is done when the edges are light golden and the center is set (it should not jiggle when shaken).
Step 9: Remove from the oven and let rest for 5-10 minutes. Garnish with fresh chopped dill, slice into squares, and serve warm.