1. Preheat oven to 350º.
2. Heat olive oil in a medium skillet over medium heat. Add leek, garlic, oregano, salt, crushed red pepper, and black pepper; cook and stir until leek is soft (about 5 minutes). Remove from heat and add the spinach, stirring to combine; set spinach mixture aside.
2. Cut the chicken breasts in half horizontally using a sharp knife. Place bottom halves of breasts in a 9- x 9-inch baking dish, setting top halves aside. Divide spinach filling evenly among breasts; top each portion with one-fourth of roasted red peppers, and replace the top halves of breasts.
3. Brush the top half of each chicken breast with melted butter; sprinkle with breadcrumbs, and spray lightly with cooking spray. Add wine to baking dish (avoiding the breadcrumbs), and bake for 40 minutes or until the chicken is completely cooked.
4. Remove chicken from oven; let rest for 5 minutes before serving. To serve, cut each breast into 3 or 4 slices on the diagonal to expose the stuffing, or simply leave each portion whole.
Slicing the chicken breast horizontally holds the filling well, and does away with the need to pin it together with toothpicks.