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.
Cook brussels sprouts in large pot of boiling salted water until crisp-tender, about 5 minutes. Drain. Place in bowl of ice water to cool. Drain and cut in half. (Mustard seeds and brussels sprouts can be made 1 day ahead. Store mustard seeds at room temperature. Wrap brussels sprouts in paper towels; cover and chill.)
Melt butter in large nonstick skillet over medium-high heat. Add shallots and sauté until tender and golden, about 4 minutes. Add brussels sprouts and sauté until just tender and heated through, about 8 minutes. Add lemon juice, mustard, and mustard seeds; toss to blend. Season with salt and pepper and serve.
