Chopped and cooked gently with cream, spinach loses its bitterness, melding with mild pearl onions and smoky bacon.
Heat a large pot over high heat until hot. Cook spinach (with water still clinging to leaves) in pot, covered, until beginning to wilt, about 1 minute. Stir, and cook, covered, 1 minute more. Uncover; stir spinach until completely wilted.
Transfer to a colander; rinse under cold water. Squeeze spinach dry in a clean kitchen towel. Finely chop; set aside.
Bring a medium saucepan of water to a boil. Add onions; cook until skins soften, about 3 minutes. Remove onions with a slotted spoon (reserve water in pot); rinse under cold water. Trim root ends, and remove skins. Halve onions if large.
Return onions to cooking water. Cook until tender, about 10 minutes. Drain, and rinse under cold water.
Melt 4 tablespoons butter in a medium saucepan over medium heat. Add flour, and whisk until smooth. Whisking constantly, pour in milk in a slow, steady stream. Boil, whisking constantly, 1 minute. Remove from heat.
Melt remaining tablespoon butter in a large saucepan over medium heat. Add bacon; cook, stirring occasionally, until well browned, about 6 minutes. Stir in onions and spinach. Stir in cream and reserved milk mixture. Add 1 teaspoon salt, 1/4 teaspoon pepper, and nutmeg. Cook, stirring occasionally, until heated through and thickened, about 10 minutes (adjust heat if necessary; do not let boil). Stir in lemon juice. Season with salt and pepper, if desired.
Slab bacon can be cut into neat little cubes, but presliced bacon works, too.