Begin by preheating your oven to 375°F (190°C). Prepare a baking sheet by lining it with parchment paper. This not only prevents sticking but also makes cleanup a breeze. I learned this the hard way after spending too much time scrubbing pans.

Finely chop the reserved mushroom stems. In a skillet over medium heat, sauté the stems with the minced garlic until fragrant and tender. I once made the mistake of using the stems whole, which resulted in a chunky texture that detracted from the overall experience.

Add the chopped spinach to the skillet and cook until wilted. Here's where I emphasize the importance of draining. Place the cooked spinach in a colander and press out as much liquid as possible. Excess moisture is the enemy of the perfect stuffing, leading to a less than desirable, watery filling.

In a bowl, combine the softened cream cheese, mozzarella, Parmesan, and feta. Mix until well blended. The cream cheese acts as a creamy binder that melds the flavors and textures of the other cheeses into a cohesive filling.

Fold in the crumbled bacon, sautéed mushroom stems, and spinach. Season with nutmeg, salt, and black pepper. The nutmeg is my secret weapon; it adds a layer of warmth that complements the cheeses and bacon beautifully. Be judicious with it, though—a little goes a long way.

Stuff each mushroom cap generously with the cheese mixture. I've found that using a small spoon or even a piping bag gives you more control and creates a neater presentation. Overfilling is better than underfilling in this case, as the mushrooms will shrink slightly as they cook.

In a small bowl, toss the panko breadcrumbs with a drizzle of olive oil. This will help them brown and crisp up in the oven. Sprinkle the breadcrumbs over the stuffed mushrooms. I once skipped the oil, and the breadcrumbs remained pale and unappealing.

Bake for 20-25 minutes, or until the mushrooms are tender and the tops are golden brown. Keep a watchful eye on them after the 15-minute mark. Oven temperatures can vary, and there's a fine line between perfectly golden and disappointingly burnt.