Preheat your oven to 375°F (190°C). This ensures the oven is at the correct temperature when you’re ready to bake the mushrooms.

Clean the mushrooms: Gently wipe each mushroom cap with a damp paper towel to remove any dirt or debris. Avoid washing them under running water, as they can absorb excess moisture and become soggy.

Remove the stems: Carefully twist or gently cut out the stems from each mushroom cap. Set the stems aside, as they will be used in the filling.

Prepare the mushroom caps: Using a small spoon, gently scrape out the brown gills from underneath the mushroom caps. This step is optional but helps to create more space for the filling and prevents the mushrooms from becoming too watery during baking.

Brush with olive oil: Lightly brush the inside and outside of each mushroom cap with olive oil. This helps to prevent them from drying out during baking and adds flavor.

Season the caps: Sprinkle the inside of each mushroom cap with salt and pepper.

Chop the mushroom stems: Finely chop the reserved mushroom stems.

Sauté the onion and mushroom stems: Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the diced onion and chopped mushroom stems and sauté for 5-7 minutes, or until the onion is softened and translucent.

Add the garlic: Add the minced garlic to the skillet and sauté for another minute until fragrant, being careful not to burn it.

Add the spinach: Add the fresh spinach to the skillet in batches, allowing it to wilt down as you add more. Stir until all the spinach is wilted and bright green. This will only take a few minutes.

Remove excess moisture: Cook off any excess moisture from the spinach mixture. Continue to cook for a couple of minutes, stirring occasionally, until most of the liquid has evaporated. This step is crucial to prevent soggy stuffed mushrooms.

Transfer to a bowl: Remove the skillet from the heat and transfer the spinach and mushroom mixture to a large mixing bowl.

Add the remaining filling ingredients: Add the crumbled feta cheese, softened cream cheese, breadcrumbs, grated Parmesan cheese, dried oregano, dried thyme, and red pepper flakes (if using) to the bowl with the spinach mixture.

Season the filling: Season the filling with salt and pepper to taste. Remember that feta cheese is already salty, so taste before adding too much salt.

Combine thoroughly: Mix all the filling ingredients together until well combined and evenly distributed.

Stuff the mushroom caps: Spoon the spinach and feta filling generously into each prepared mushroom cap, mounding it slightly.

Arrange on a baking sheet: Place the stuffed mushroom caps on a baking sheet lined with parchment paper or foil for easy cleanup.

Bake: Bake in the preheated oven for 20-25 minutes, or until the mushrooms are tender and the filling is heated through and lightly golden brown on top.

Optional Broiling (for extra browning): For extra browning on top, you can broil the mushrooms for the last 1-2 minutes of baking. Watch them closely to prevent burning.

Rest: Let the stuffed mushrooms rest for a few minutes before serving. This allows the filling to set slightly and the mushrooms to cool down enough to handle.

Garnish (optional): Garnish the baked stuffed mushrooms with a sprinkle of fresh parsley or chives for added freshness and visual appeal.

Serve warm: Serve the Spinach and Feta Stuffed Mushrooms warm as an appetizer, side dish, or light meal.