Preheat oven to 400°F (200°C).
Prepare chicken:
Slice each chicken breast horizontally to create a pocket, starting at the thickest part. Don't cut all the way through.
Season with salt, pepper, and oregano.
Stuff chicken:
Place 2 tomato slices, 2 mozzarella slices, and a few spinach leaves in each pocket. Secure with 4 toothpicks diagonally.
Sear:
Heat 2 tsp olive oil in a pan over medium-high heat. Sear chicken for 2 minutes per side, flipping carefully.
Bake:
Transfer chicken to a casserole dish (or use the pan if oven-safe). Bake for 15 minutes or until the chicken is cooked through (no longer pink, juices run clear).
Serve:
Remove toothpicks and enjoy!