Preheat oven to 350 °.

Heat olive oil in an 8 inch cast iron skillet or a heat proof pan on a medium low heat until hot.

Add sausage and onions and cook on low until warmed through.

Meanwhile crack 12 large eggs into a big bowl. Add milk, salt and pepper. Whisk until well incorporated. Add the thyme and 1/2 cup of cheese. Whisk to mix together.

Add the egg mixture to the pan and top with the asparagus. Cook for 5 minutes on medium low heat until the edges set. Do not stir. Add the remaining cheese to the top of the frittata and bake in the oven for 25-30 minutes. Do NOT overbake. The custard should be set, but still a little jiggly.

Let the frittata sit for 5 minutes. Slice and serve.