Trim the dark green tops and root ends. Slice the leeks crosswise into 1-inch pieces. Check the cut sides and, if an outer ring looks tough or dirt is trapped between layers, trim away just that outer section and discard. Transfer the sliced leeks to a bowl of cold water and let sit several minutes to loosen any remaining grit. Lift out and pat very dry.

Preheat the oven to 325 degrees F with a rack in the top third. Use your fingers to butter a small 1-1 ½-quart baking dish (such as a 9-inch oval or 6×8-inch rectangular dish), then season lightly with kosher salt and a few twists of black pepper. Place on a small baking sheet to catch any drips, then set aside.

Add just enough oil to lightly coat the bottom of a small skillet and place over medium heat. When hot, arrange the leeks in a single layer, cut side down, and cook 4-5 minutes until golden. Flip and cook on the second cut side for 4-5 minutes more. Transfer to the prepared baking dish, arranging them flat so the seared cut sides face up and down (not on their rounded sides), leaving a little space between pieces for the cream sauce.

Deglaze the skillet used to sear the leeks with white wine, scraping up any browned bits from the bottom of the pan. Simmer until reduced by about half. Reduce the heat to medium-low and stir in the cream, Dijon, and fresh thyme leaves. Season with a scant ½ teaspoon kosher salt, plus a few twists of black pepper. Gently simmer 2-3 minutes, until the sauce is slightly thickened and coats the back of a spoon. Avoid boiling, which can cause the cream to separate.

Pour the thickened cream sauce over the leeks. The liquid should come nearly to the tops of the leeks without fully submerging them. Cover the baking dish tightly with foil. Transfer to the oven and bake for 35-40 minutes, until the leeks are very tender and easily pierced with a fork.

Optional: Quickly finish under the broiler: Remove the foil and sprinkle parmesan over the creamy braised leeks. Turn the broiler to high. Return the leeks to the oven and broil 3-5 minutes, until bubbly and golden on top.

Allow to cool slightly, then finish with a showering of finely chopped fresh chives, if desired. Enjoy!