Prepare the Mushrooms: Clean the mushrooms thoroughly, then slice them into pieces.
Heat the Oil: Place a large sauté pan or skillet over medium-high heat and add the olive oil, allowing it to heat before adding the mushrooms.
Sauté the Mushrooms: Add the sliced mushrooms and cook for 4 minutes, stirring often so they cook evenly.
Season the Mushrooms: Sprinkle in the kosher salt and add a few grinds of fresh black pepper, then continue cooking for 2 minutes, stirring occasionally, until most of the liquid released by the mushrooms has cooked out and the mushrooms are tender.
Cook the Garlic: Add the minced garlic to the pan and sauté for 1 minute, stirring so the garlic combines with the mushrooms.
Wilt the Spinach: Add the baby spinach leaves and cook for 2 minutes, stirring as needed, until the spinach has wilted.
Add the Lemon Juice: Turn off the heat, then add the fresh lemon juice along with 2 pinches of salt and stir thoroughly to distribute the seasoning.
Adjust the Seasoning: Taste the sautéed vegetables and add more salt or freshly ground black pepper as needed.
Garnish and Serve: If desired, finish with a sprinkle of grated Parmesan cheese and serve immediately.
