Preheat the oven - Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper.

Prepare the mushrooms - Wipe the Portobello mushrooms clean with a damp towel and carefully remove the stems. Using a spoon, gently scrape out the gills from the underside of the caps to create more room for the filling. Place the mushrooms on the prepared baking sheet. - Be careful not to break the edges of the mushroom caps while removing the stems and gills, as this will make it harder to contain the filling.

Cook the spinach and garlic - In a large skillet, heat 1 tablespoon of olive oil over medium heat. Add minced garlic and cook for 1-2 minutes until fragrant. Add baby spinach to the skillet and cook for 2-3 minutes until wilted. Remove from heat and set aside.

Make the filling - In a mixing bowl, combine cream cheese, grated Parmesan cheese, and cooked spinach and garlic. Mix well until combined, and season with salt and black pepper to taste.

Stuff the mushrooms - Divide the filling evenly among the mushroom caps, gently pressing it down to make sure it stays in place. In a small mixing bowl, combine breadcrumbs and 1 tablespoon of olive oil. Mix well and sprinkle over the top of each stuffed mushroom.

Bake the stuffed mushrooms - Bake the stuffed mushrooms in the preheated oven for 20-25 minutes, or until the mushrooms are tender and the topping is golden and crispy. Remove from the oven and let them cool for a few minutes before serving. - You can broil the stuffed mushrooms for an additional 1-2 minutes to get a more golden and crispy topping, but be careful not to burn them.