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  1. To make the filling: In a large skillet, heat the oil over medium heat. Add the onions and cook, stirring occasionally, until translucent (about 5 minutes). Add the spinach and bread crumbs and stir until well combined. Remove from the heat, transfer to a bowl, and let cool to lukewarm.

  2. Stir in the cheeses and peppers and season with salt and pepper.

  3. Line two baking sheets with parchment.

  4. To assemble: Roll the dough into a 12" x 18" rectangle. Trim 1/4" of dough from the edges; this will give you a higher puff in the oven.

  5. Cut the dough in thirds in both directions, to make nine 4" x 6" rectangles. Place 2 tablespoons of the filling on one side of each rectangle.

  6. Fold the dough over twice to enclose the filling, with the seam ending up on the bottom. Press the croissant down slightly to flatten it.

  7. Place the croissants on the prepared baking sheets and let them rise for 1 to 1 1/2 hours at room temperature. They should expand noticeably, and when you gently press one with your finger, the indentation should remain.

  8. Partway through the rise time, preheat the oven to 425°F.

  9. When the croissants are not quite doubled, brush with the egg wash and bake for 15 minutes. Reduce the oven temperature to 350°F and bake for 15 minutes more, until the croissants are a deep golden brown, even where the dough overlaps (that's where you want to look when checking for doneness).

  10. Remove from the oven and cool on a rack for 20 minutes before serving. Watch out, the filling will be hot!

  11. Store, wrapped in the refrigerator, for up to two days. Refresh for 10 minutes in a 350°F oven before serving.

Tips from our Bakers

  • This recipe makes enough filling for 18 croissants. You can make and use a whole batch of croissant dough to match the filling, and freeze half of the shaped croissants, unbaked, if you like. Thaw frozen croissants in the refrigerator overnight before bringing to room temperature for their second rise, then bake as directed.
  • Extra spinach filling is excellent for making calzones or as pizza topping.