Zest lemon. Remove lemon pith with a sharp knife and slice flesh horizontally. Reserve slices.

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.

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.

Preheat the oven to 350 degrees F (175 degrees C).

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.

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.

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.

Bake in the preheated oven until an instant-read thermometer inserted into the center registers 165 degrees F (74 degrees C), about 35 minutes.