Trim the stem ends of the Brussels sprouts and remove any loose or discolored leaves.

Bring a large saucepan of water to a boil. Add a generouse pinch of salt and the sprouts and cook, uncovered over medium heat until the sprouts are tender but still firm to the bite, 8 to 10 minutes. Drain and immediately plunge them into a large bowl of ice water to stop the cooking and set their green color. Drian the sprouts, pat dry with paper towels and place in a large bowl.

Preheat the oven to 350 degrees.

Melt the butter in a small skillet over medium heat. Add the shallots and cook, stirrring a few times, until soft, 4 to 5 minutes. Stir in the pancetta and cook until it just begins to color, 1 to 2 minutes. Pur the contents of the skillet over the sprouts, add about half of the Parmigiano, and season with salt. Mix well to combine.

Generously butter a large baking pan, add the sprouts in a single layer if possible and sprinkle the remaining Parmigiano over them.

Put the pan on the middle rack of the oven and bake until the cheese is melted and light golden, 10 to 12 minutes. Serve hot.