Blue cheese béchamel

1. Gradually heat milk with thyme, peppercorns, garlic, and onion in a small saucepan until just simmering. Turn off heat and strain milk into a liquid measuring cup. In a medium saucepan, melt butter over medium, then add flour and stir 3 to 5 minutes, moving on to the next step if the mixture begins to darken. Reduce heat to medium low and whisk while adding warm milk. Once combined, increase heat gradually until sauce simmers, then maintain a simmer and stir almost constantly until thickened, 5 to 10 minutes. Off heat, whisk in gorgonzola until nearly melted, then stir in cream. Season with a pinch of ground or grated nutmeg and salt and pepper to taste. Cover or otherwise keep warm while assembling the gratin.

Gratin assembly

1. Preheat oven to 375 (F). Set a medium pot of water over high heat. Meanwhile, rub a 2 qt. gratin or baking dish with the cut surface of the garlic clove, then grease with butter.
2. When water boils, add a generous pinch of salt, then the leeks. Keep heat on high as the leeks cook for 2 minutes, then remove to a colander set over a dish towel. Place turnips in boiling water and cook 4 minutes, moving leeks to a small bowl in the meantime. Drain turnips in the colander. Make a layer of half the turnips followed by half the leeks and all the thyme sprigs in the prepared dish, then sprinkle with salt and pepper. Layer remaining turnips and leeks over the top, followed by more salt and pepper. Pour béchamel over the vegetables to cover, then gently shake the pan to work sauce down into the vegetable layers.
3. Bake 30 minutes just above the center rack in the oven. If desired, run under the broiler for a couple minutes to achieve golden spots on top. Cool 5 minutes before serving. Gratin can be assembled a day ahead without baking, and leftovers keep well in the refrigerator almost a week.