Roughly chop spinach leaves; set aside. Combine milk, peppercorns, bay leaves, and cloves in a small saucepan. Warm over medium heat until hot but not boiling. Remove from heat, cover, and let steep for 30 minutes. Strain into a bowl or measuring cup, discarding solids; set aside.
Melt 1 tablespoon butter in a saucepan over medium heat. Whisk in flour; cook, whisking constantly for 1 minute. Gradually whisk in milk mixture; cook, whisking constantly until mixture thickens, about 3 minutes. Stir in salt, white pepper, nutmeg, and half-and-half. Set aside, covered; keep warm over low heat.
Heat remaining butter in a large, nonstick skillet over medium-low heat. Add onion; cook until tender, about 5 minutes. Add spinach a little at a time; turn with tongs until all spinach is added and just wilted. Pour sauce over spinach, and stir to combine. Serve immediately.