1. Cook bacon in a large cast-iron skillet over medium-high heat until crisp. Remove bacon from pan, reserving 1 tablespoon drippings in pan; coarsely chop and set bacon aside. Increase heat to high.
2. Pat scallops dry with paper towels. Sprinkle scallops evenly with 1/4 teaspoon salt and 1/8 teaspoon pepper. Add scallops to drippings in pan; cook 2 1/2 minutes on each side or until done. Transfer to a plate; keep warm. Reduce heat to medium-high. Add onion and garlic to pan; sauté 3 minutes, stirring frequently. Add half of spinach; cook 1 minute, stirring frequently. Add remaining half of spinach; cook 2 minutes or just until wilted, stirring frequently. Remove from heat; stir in remaining 1/8 teaspoon salt and remaining 1/8 teaspoon pepper. Divide spinach mixture among 4 plates; top each serving evenly with crumbled bacon and 3 scallops. Serve immediately with lemon wedges, if desired.