How to Make It
Preheat the oven to 400°. Butter a large cast-iron skillet or casserole dish and set it aside.
Bring a large pot of salted water to a boil. Add the pasta and cook, following the package directions, until 3 minutes shy of al dente. The pasta should still have a bite to it. Drain and shake out all the water.
Meanwhile, in a large pot or Dutch oven, heat the oil. Sautée the shallot and radicchio over medium-high heat until wilted, 3 minutes. Remove to a bowl.
In the same pot, melt the butter. Whisk in the flour and cook until it is fully incorporated, 1 minute. Add the warm milk and whisk gently over medium-high heat until the mixture has bubbled and thickened enough to coat the back of a spoon, about 10 to 15 minutes. Off the heat, stir in the salt, raisins and cheese; stir until melted. Add the pasta and radicchio and toss to combine.
Distribute the mixture evenly in the prepared skillet or baking dish. Bake for 20 to 25 minutes, until bubbling and browned. Serve piping hot.