Preheat your oven to 375°F
In a large mixing bowl, whisk together the eggs and milk or cream. Season with salt and pepper and set aside.
Heat olive oil in a large oven-safe skillet over medium heat. Add the diced onion, asparagus, broccolini and scallion. Sauté for about 5-7 minutes until the vegetables are tender.
Pour the egg mixture over the cooked vegetables in the skillet. Gently stir the mixture to distribute the vegetables evenly.
Let the bottom of the frittata set on the stove on medium to low heat (making sure it doesn't burn), about 3-5 min.
Place the skillet in the preheated oven and bake for 15-20 minutes or until the eggs are set and the cheese is melted and lightly golden.
Once cooked, remove the frittata from the oven and let it cool for a few minutes.
Combine vinaigrette ingredients and pour over top frittata.
Garnish with fresh herbs, and enjoy!