Brown Butter Creamed Winter Greens

• 3/4 stick unsalted butter, divided
• 2 tablespoons all-purpose flour
• 2 cups whole milk
• 2 tablespoons minced shallot
• 1 Turkish or 1/2 California bay leaf
• 6 black peppercorns
• 3 1/2 pounds mixed winter greens such as collards, mustard greens, and kale
• 6 ounces slab bacon, any rind discarded and bacon cut into 1/4-inch-thick slices, then cut crosswise into 1/4-inch sticks (optional)
• 1 cup finely chopped onion
• 1/2 cup heavy cream
• 2 garlic cloves, minced
• 1 teaspoon dried hot red-pepper flakes
• 1 tablespoon cider vinegar, or to taste

Melt 2 tablespoons butter in a heavy medium saucepan over medium heat, then add flour and cook, stirring, 1 minute.

Add milk in a stream, whisking, then add shallot, bay leaf, and peppercorns and bring to a boil, whisking. Simmer, whisking occasionally, 5 minutes. Strain béchamel sauce through a fine-mesh sieve into a bowl, discarding solids, and cover surface with parchment paper.

Discard stems and center ribs from greens, then coarsely chop leaves.

Cook lardons in a wide 6- to 8-quart heavy pot over medium heat, stirring occasionally, until golden-brown but not crisp, about 8 minutes. Transfer to paper towels to drain, then pour off fat from pot and wipe clean.

Heat remaining 1/2 stick butter in pot over medium-low heat until browned and fragrant, about 2 minutes, then cook onion, stirring, until softened, about 3 minutes.

Increase heat to medium-high, then stir in greens, 1 handful at a time, letting each handful wilt before adding next. Add béchamel, cream, garlic, red-pepper flakes, 3/4 teaspoon salt, and 1/2 teaspoon pepper and boil, uncovered, stirring, until sauce coats greens and greens are tender, about 10 minutes.

Stir in lardons, vinegar, and salt and pepper to taste.