Peel and thinly slice the onion. Cut the zucchini piece length wise and then into slices to get half-moons. Slice the yellow bell pepper into strips, discarding the seeds.
In an oven-safe, heavy pan, heat the oil.
Add the sliced vegetables, except the greens. Sautxe9 on medium flame for 3-4 minutes.
Add the chopped greens and sautxe9 for 30 seconds until wilted.
Meanwhile, crack the eggs into a large bowl. Whisk well, along with milk, salt, pepper and herbs.
Preheat the oven at 175xb0C.
Spread out the sautxe9ed vegetables in the pan in a single layer. Pour the egg mixture over the spread out vegetables. With a sharp spatula, keep making spaces in the frittata so the cooked egg portion moves aside and the liquid eggs reach the bottom of the pan to get cooked. Repeat this procedure all around the pan for 4-5 minutes, while the pan is on medium flame. Once the eggs are set at the bottom and nearly setting at the top, sprinkle the cheese over the top.
Place the pan into the oven and bake for 5-7 minutes, until the top is fully set and cheese has melted.
Cut into 4 wedges and serve with toast. This tastes good cold as well, inside a sandwich.