To make flavored milk:
Combine milk, peppercorns, bay leaves and cloves in small saucepan. Warm over medium heat until hot but not boiling.
Remove from heat, cover, and let sit for 20 minutes.
Strain into small bowl or measuring cup, set aside. Discard solids.
To make Be'chamel sauce:
Melt 1 Tbs. butter in a saucepan over medium heat.
Whisk in flour; cook for 1 minute, whisking constantly.
Gradually whisk in milk mixture. Whisk another 3 minutes until sauce thickens.
Stir in salt, white pepper, nutmeg and half & half. Set aside, covered.
To finish the dish:
Heat remaining butter and a splash of olive oil in a large skillet over medium heat.
Add onion, sauté until tender, about 5 minutes.
Add 1/2 bag of spinach; cover skillet momentarily until all leaves are wilt.
Add the other half bag. Turn with tongs until all leaves are wilted.
Pour sauce over spinach, stir to combine. Serve.