Stir mustard seeds in small dry skillet over medium-low heat until seeds are lightly toasted and begin to pop, about 3 minutes. Remove from heat. Cool.
Melt butter in large nonstick skillet over medium-high heat. Add shallots and sauté until tender and golden. Add brussels sprouts and cover, checking occasionally and adding a small amount of water to prevent sticking. When just tender, remove the lid and sauté until the sprouts are browning in places. Add lemon juice, mustard, and mustard seeds; toss to blend. Season with salt and pepper and serve.
Adapted from Bon Appétit, December 2001