Halve the Brussels sprouts lengthwise and them thinly slice crosswise, discarding ends that may be slightly brown. In a medium-sized sauté pan, melt the butter over medium-high heat. Add the shallots and Brussels sprouts and sauté until crisp-tender, about 4 minutes. Add the lemon zest, lemon juice and hazelnuts. Season with salt and pepper. Serves 6.