How to Make It
Preheat the oven to 350°. Butter a 9-inch square baking dish. In a large saucepan, melt the 1 stick of butter. Add the 1/2 cup of flour and whisk until smooth. Cook over moderate heat, whisking, for 2 minutes, then gradually whisk in the milk and simmer until very thick, about 5 minutes. Add the Gruyère, robiola and yogurt and cook over very low heat, just until melted. Stir in the nutmeg and season with salt and pepper. Keep warm.
In a large pot of salted boiling water, cook the pasta until barely al dente. Drain and return to the pot. Add the cheese sauce and stir to combine. Scrape the pasta into the prepared baking dish and smooth the top. Cover the dish with foil and bake for 20 minutes. Let cool, then refrigerate until completely chilled, at least 4 hours.
Cut the baked ziti in half, then cut each half crosswise into 8 slabs. Dust each slab with flour. Lightly coat the bottom of a large nonstick skillet with oil. Working in batches, fry the slabs over high heat, turning once, until browned on the outside and hot within, about 5 minutes.
Arrange 2 slabs in a shallow bowl and spoon the warm Lamb Sausage Ragù on top. Sprinkle with Pecorino and garnish with mint.