Preheat oven to 375F and line a 9×13 inch pan with parchment paper.
In a preheated saucepan, saute the onions, garlic, and greens. Feel free to add other vegetables or crumbled sausage in this step as well.
In a large bowl, whisk together the eggs, salt, and milk.
Once the sauteed vegetables have softened, whisk them into the eggs.
Pour the mixture into the pan and bake for 20-30 minutes or until cooked through and puffed.
Let cool for a few minutes and then cut into squares or circles to match the bread you are using.
Slather bread slices with pesto, top with a slice of frittata, cheese, tomato, avocado, and arugula if using.
Pack up and take with you on the go!