Preheat oven to 375°F. In a 6" cast iron skillet, heat olive oil over medium heat. Add onion and cook for about 5 minutes or until translucent. Add garlic and cook for another minute.
While cooking onion, whisk together eggs with milk and salt. Add in kale, roasted red peppers, Parmesan cheese and pesto. Add egg mixture to skillet and cook until bottom begins to set, about 5 minutes.
Transfer skillet to oven and cook for another 8-10 minutes if using an 8" skillet or 20-25 minutes if using a 6" inch skillet, or until top is set. Remove from oven and top with some fresh basil.
Use whole full fat dairy which gives the frittata its flavor. You can substitute milk for yogurt or sour cream.
Don't mess with the egg to dairy ratio or you won't get the right consistency.
Cast iron skillets are the only way to go for a frittata. Don't consider anything else.
You know the frittata is done when the center is no longer jiggly and the edges are golden brown.