Heat olive oil or butter in a cast iron skillet over medium heat, then sauté the onions and garlic until the onions are soft and translucent, about 5-7 minutes.
Add the asparagus and season with salt and pepper. Cook until veggies are just tender, about 5 minutes, then remove from the stovetop and transfer to a large plate or bowl to cool.
While the veggies are cooling, whisk together eggs, cooking cream (or milk), and mustard in a bowl.
Add the cooled vegetables into the egg mixture.