Step 1
Zest lemon. Remove lemon pith with a sharp knife and slice flesh horizontally. Reserve slices.
Step 2
Heat 2 tablespoons olive oil in an oven-safe nonstick skillet over medium heat until it shimmers 2 to 3 minutes. Add onion and cook until translucent 3 to 5 minutes. Add lemon zest and garlic and heat until fragrant about 1 minute.
Step 3
Add spinach to the onion mixture. Cook and stir until heated through about 3 minutes. Remove spinach-onion mixture to a bowl. Wipe skillet clean and set aside. Stir feta cheese and olives into the spinach mixture. Season with salt and pepper.
Step 4
Preheat the oven to 350 degrees F (175 degrees C).
Step 5
Cut a 2-inch slit along the edges of the thick side of each chicken breast. Cut lengthwise being careful not to cut through the bottom or the opposite edge creating a pocket. Season the pocket with salt and pepper.
Step 6
Stuff each chicken breast with 1/2 of the onion and spinach mixture and close the slits with toothpicks. Dust chicken lightly with flour salt and pepper.
Step 7
Heat 2 tablespoons olive oil in the same skillet over medium-high heat until shimmering about 2 minutes. Place chicken in the hot oil with the rib side facing up. Cook until browned 3 to 5 minutes per side. Turn chicken rib-side down and place lemon slices on top.
Step 8
Bake in the preheated oven until an instant-read thermometer inserted into the center registers 165 degrees F (74 degrees C) about 35 minutes.