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 bacon, and set aside. Increase heat to high.
2. Pat scallops dry with paper towels. Sprinkle scallops evenly with 1/8 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 chopped bacon and 3 scallops. Serve immediately with lemon wedges, if desired.
Scallops cooked in a pan are always in danger of being steamed rather than seared, so be sure to use a big pan and high heat. A cast-iron skillet is ideal since it gets really hot and can handle high temperatures, unlike a nonstick skillet, which has a more delicate nonstick coating that can only handle up to medium-high heat.