1 pound medium beets
3 sprigs fresh thyme branches plus 3/4 teaspoon fresh thyme leaves
2 garlic cloves, peeled
¼ teaspoon black peppercorns
Kosher salt and black pepper
1 ¼ pounds beet greens,11 tablespoons cold unsalted butter, more for greasing pan Swiss chard or a mix of both
1 1/3 cups all-purpose flour
2 cups milk
9 ounces sharp Cheddar, grated (2 1/4 cups)
2 to 3 tablespoons English mustard powder, to taste
1 teaspoon Worcestershire sauce
½ teaspoon Tabasco sauce, more as needed
1/4 cup rolled oats
3 tablespoons toasted cashews, chopped
1 ½ teaspoons grated Parmigiano-Reggiano
¼ teaspoon freshly grated nutmeg
Step 1: Combine beets, thyme branches, garlic and peppercorns in a large pot. Cover with cold salted water. Bring to a simmer over medium-high heat; cook until beets are tender, 15 to 30 minutes depending on size. Remove with a slotted spoon. Add greens and cook for 2 minutes (do this in batches if necessary); remove with tongs and transfer immediately to a bowl of ice water. Drain well.
Step 2: Once beets are cool enough to handle, peel them and slice crosswise into 1/4-inch-thick slices. Roughly chop greens’ leaves and stalks.
Step 3: Prepare the béchamel: In a small saucepan over low heat, melt 5 tablespoons butter. Stir in 75 grams flour ( 2/3 cup). Cook, stirring constantly, for 2 minutes; roux should smell cooked but remain white. Slowly whisk in milk until mixture forms a thick, smooth sauce. Stir in 8 ounces Cheddar (2 cups) until melted. Stir in mustard powder, Worcestershire and Tabasco. Season with salt to taste.
Step 4: Make the crumble topping: In a small bowl, stir together remaining 2/3 cup flour, the oats and the cashews. Use your fingers to work in 6 tablespoons butter, the remaining 1 ounce Cheddar ( 1/4 cup) and the Parmigiano-Reggiano. It should be a mixture of large and small pieces. Season with a pinch of salt, pepper and the nutmeg.
Step 5: When you are ready to assemble the dish, heat oven to 350 degrees.
Step 6: Butter a 2-quart gratin or baking dish. Spread a layer of béchamel on the bottom. Top with a layer of beets, followed by a layer of greens and stalks. Season generously with salt and pepper, and sprinkle with thyme leaves.
Step 7: Spread another layer of béchamel on top and repeat process to fill dish (you will end up with 3 or 4 layers).
Step 8: Cover entire surface with crumble topping. Transfer dish to oven and bake until bubbling and golden brown in spots, 45 minutes to 1 hour.