Step 1: Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add 2 tablespoons (rinsed) mustard seeds and cook, stirring frequently, until seeds begin to pop.
Step 2: Add 2 large sliced shallots and 1/4 teaspoon salt and toss to coat. Lower heat to medium and cook, stirring frequently, until lightly browned, 4 minutes. Step 3: Add 2 ½ cups shredded brussels sprouts and 1/2 teaspoon salt. Pour in 1/3 cup water, reduce heat to medium, and cook until sprouts are crisp-tender, about 4 minutes. Stir in 2 tablespoons lemon juice. Season with more salt, if desired. Toss in ½ cup chopped walnuts and serve.