Preheat oven to 400 degrees F (200 degrees C). Line a baking sheet with parchment paper.
Place chicken in a bowl with garlic, parsley, lemon juice, and pepper.
Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned and crispy, about 5 minutes. Drain bacon slices on paper towels, reserving grease in the skillet.
Saute chicken in the bacon grease until no longer pink on the outside, 5 to 7 minutes. Add onion; cook and stir until onion is translucent, about 5 minutes. Add spinach to pan, 1 handful at a time, until it begins to wilt, about 3 minutes. Remove from heat.
Cut each pastry sheet in half. Spread out pastry on a floured surface. Layer a portion of feta cheese, chicken, and bacon-spinach mixture over each piece. Fold pastry over stuffing and crimp edges with fingers. Place pockets seam-side down on the prepared baking sheet.
Bake in the preheated oven until golden brown, about 30 minutes.