Step 1: Prepare the Filling

1. Heat the olive oil in a large skillet over medium heat. Add the chopped onion and cook for 3–4 minutes, stirring frequently until softened and translucent.
2. Add the minced garlic and cook for another minute, until fragrant.
3. Add the mushrooms to the skillet and sauté for 5–6 minutes, until they release their moisture and become golden brown.
4. Toss in the chopped spinach and stir until wilted, about 2 minutes.
5. Season with rosemary, salt, and pepper, then remove from heat and let the mixture cool for a few minutes.
6. Once cooled, transfer to a large bowl and fold in the feta cheese and beaten egg. Mix until well combined and set aside.

Step 2: Prepare the Phyllo Pastry

1. Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.
2. On a clean, dry surface, lay out one sheet of phyllo pastry and brush it lightly with melted butter. Place another sheet on top and brush again, repeating until you have a stack of three sheets.
3. Cut the phyllo stack into 4-inch squares (you should get about 8–10 squares from a single phyllo stack).

Step 3: Fill and Shape the Bites

1. Place a heaping tablespoon of the mushroom and feta filling in the center of each phyllo square.
2. Gather the corners of the square towards the center, gently pinching to seal and form a small bundle. Place each bundle onto the prepared baking sheet.
3. Repeat the process with the remaining phyllo and filling until all of the filling is used.

Step 4: Bake the Bites

1. Brush the tops of each phyllo bundle with a bit more melted butter to help them turn golden brown.
2. Bake for 18–20 minutes, or until the phyllo pastry is crisp and golden.

Step 5: Cool and Serve

1. Remove the bites from the oven and let them cool on the baking sheet for 5 minutes before serving. These are best enjoyed warm but can also be served at room temperature.