Place your large sauté pan over medium-high heat and add the olive oil and butter. Add the sliced brown cremini mushrooms in a single layer and cook undisturbed for 3-4 minutes until they release their moisture and develop a deep brown crust.

Toss the fresh spinach into the pan and stir constantly for about 60 seconds until you see wilted dark green spinach leaves. Remove from heat, let cool slightly, and drain any excess liquid.

Preheat your oven to 375°F (190°C) and thoroughly grease a 12-cup muffin tin. In a large mixing bowl, vigorously whisk the eggs, heavy cream, baking powder, and kosher salt until perfectly uniform and frothy.

Gently fold the cooled mushroom and spinach mixture, along with the shredded Gruyère cheese, directly into the egg base until evenly distributed.

Divide the mixture evenly among the muffin cups, filling each 3/4 full. Bake for 18-20 minutes until you see a slightly puffed baked egg base with golden-brown crispy edges pulling away from the tin.

Remove from the oven, immediately garnish with a sprinkle of cracked black pepper and fresh parsley flakes, and let rest for 5 minutes before transferring to a wire rack.