Preheat oven to 425°F.

Whisk together eggs, dairy, and salt until egg yolks and whites are blended (do not over-whisk). Whisk in half of the cheese.

If using raw vegetables, warm olive oil over medium heat in a 12-inch cast-iron skillet or oven-safe nonstick skillet. Add raw vegetables and cook until tender, then add any garlic or greens, and cook until fragrant or wilted. Season with salt, to taste.

If using precooked veggies or leftovers, add them to the heated skillet. Stir in 2 tablespoons of water and cook until warm.

Whisk the eggs once more and pour the mixture over vegetables. Stir with a spatula briefly to combine and distribute the mixture evenly across the pan. Sprinkle the other half of the cheese on top of the frittata.

Once the outside edge of the frittata turns lighter in color, about 1 minute, carefully transfer the pan to the oven. Bake for 7–14 minutes, keeping an eye on it and waiting until the eggs are puffy and appear cooked but the center of the frittata jiggles just a bit. Remove the frittata from the oven and place it on a cooling rack. Garnish with herbs and serve.