Preheat large skillet to medium high. Add butter and olive oil to pan (butter for flavor, oil for higher smoke point). When the butter is melted, add the mushrooms in a single layer. If your skillet is not large enough to hold all the mushrooms in a single layer, cook the mushrooms in two batches so they will brown, rather than steaming.

Sprinkle the tops of the mushrooms with ¼ teaspoon kosher salt and cook without disturbing for 3 minutes to form a nice sear on the first side.

Flip mushrooms and cook with minimal stirring for an additional 2-3 minutes until browned on the other side.

Reduce heat to medium low. Add garlic and another splash of olive oil if it's looking dry, along with half the spinach. Cover for 30 seconds to wilt the greens. Stir, add the other half of the spinach, and cover for another 30 seconds.

Add remaining ¼ teaspoon salt and a squeeze of fresh lemon juice. Stir to combine and serve immediately.