Preheat the oven to 375°F (190°C). Lightly grease an ovenproof baking dish or skillet.

If roasting your own pepper: place whole red bell pepper under broiler or on a hot grill until charred on all sides, then transfer to a bowl and cover for 10 minutes. Peel, seed, and dice. (If using jarred roasted peppers, just chop and drain.)

In a medium bowl, stir together roasted red pepper, chopped spinach, mozzarella, Parmesan, breadcrumbs, beaten egg, minced garlic, Italian seasoning, salt, and pepper until evenly combined.

Butterfly each chicken breast: lay flat and slice horizontally almost through to create a pocket, or slice completely and open like a book. If needed, gently pound with a meat mallet to even thickness (about ½ inch).

Spoon an even portion of the filling into each breast, then roll or fold to enclose. Secure with toothpicks or kitchen twine if necessary.

Heat a large oven-safe skillet over medium-high heat. Add butter and/or 1 tablespoon olive oil. Sear the stuffed chicken seam-side down for 2–3 minutes until golden; flip and sear the other side for 1–2 minutes.

Brush the tops with remaining olive oil, then transfer skillet or baking dish to the preheated oven. Bake for 20–25 minutes, or until an instant-read thermometer inserted into the thickest part reads 165°F (74°C). If using a thinner cut, check at 18 minutes.

Remove from oven and let rest 5–7 minutes. Remove toothpicks or twine, slice on a bias, and serve with lemon wedges and fresh herbs.