Heat a large skillet over medium. Add pancetta; cook until crisp, 4 to 6 minutes. With a slotted spoon, transfer pancetta to a paper-towel-lined plate; reserve fat in skillet.
Add as much spinach as will fit in skillet; season with salt and pepper. Cook, tossing spinach and adding more as it wilts. Continue to cook until tender, about 2 minutes. Remove from heat, and pour off excess liquid; stir in lemon juice. Top with pancetta.