While pie crust is baking, heat oil and/or butter in a large skillet over medium heat until hot. Add onion and cook 3 minutes. Add asparagus and cook approximately 5 minutes. Add the garlic and cook another 1-2 minutes (do not let garlic brown, this will cause it to taste bitter). Add spinach and wilt, takes approximately 1-2 minutes. Remove from heat, stir in parsley.
In a medium bowl, whisk eggs until blended, whisk in milk or non-dairy alternative. Add a dash of salt and pepper. Add cheddar cheese and stir to combine.
Spread out spinach, asparagus, shallot, garlic, and parsley mixture over the bottom of the pie crust; pour in egg and cheddar cheese mixture. (If adding salmon or chicken do so now). Sprinkle Parmesan cheese on top.
Bake at 375 degrees for 30-40 minutes or until set and knife inserted in the center comes out clean. Cool on a wire rack for 10 minutes.