Blanch the spinach by boiling it in salty water for about 90 seconds, then submerge in ice water and squeeze dry. Roughly chop and set aside.

Cook the mushrooms in a cast iron pan over medium-high heat until they release their moisture and reduce. Add minced garlic and a pinch of salt, then cook until the liquid evaporates. Add butter and sauté until golden-brown.

In a bowl, beat the eggs with cream or milk. Whisk in shredded gruyere and grated pecorino, along with chopped spinach, until well combined.

Reduce the skillet heat to medium-low and pour in the egg mixture, distributing evenly over the mushrooms. Cook undisturbed for about 10 to 15 minutes or until edges are set and center is firm.

Finish the frittata under the broiler for a few minutes to brown the top, watching closely to prevent burning.

Let the frittata cool for 5 to 10 minutes before slicing into wedges. Serve warm.