Preheat your oven to 375°F (190°C) and prepare a baking sheet lined with parchment paper.

In a large skillet, heat a tablespoon of olive oil over medium heat. Add the finely chopped onions and minced garlic, sautéing for about 3-4 minutes until translucent.

Add the sliced baby bella or cremini mushrooms to the skillet and cook for approximately 5-7 minutes until softened.

Stir in the fresh spinach and let it wilt for about 2-3 minutes, seasoning with salt and pepper to taste.

In a mixing bowl, combine the cooled mushroom, spinach, and onion mixture with the cream cheese and grated cheese, blending until smooth.

On a lightly floured surface, roll out the puff pastry sheets and cut them into 24 squares, each about 3 inches by 3 inches.

Place a teaspoon of the creamy filling in the center of each pastry square and gently fold to create triangles or rectangles.

Press down to seal the edges and brush the tops lightly with a beaten egg wash for a golden finish.

Arrange the stuffed pastries on your prepared baking sheet and bake for 20-25 minutes until golden brown.

Remove from the oven and let cool on the baking sheet for about 10 minutes before serving.