Trim Brussels sprouts. Cut sprouts lengthwise in half, then cut crosswise into thin slices.
Place a large cast-iron or other heavy skillet over medium-high heat. Add Brussels sprouts; cook and stir until sprouts begin to brown lightly, 2-3 minutes. Add oil and toss to coat. Stir in onion, ginger, garlic and salt. Add water; reduce heat to medium and cook, covered, until vegetables are tender, 1-2 minutes. Stir in pepper.
Test Kitchen tips
Molasses-Sriracha variation: Add 1 Tbsp. molasses and 2 tsp. sriracha to the water and cook as directed.
Sesame-Ginger variation: Substitute toasted sesame oil for the olive oil and proceed as directed. Sprinkle 1 Tbsp. toasted sesame seeds over cooked sprouts before serving.
Cranberry-Pecan variation: Add 1/4 cup dried cranberries with the onion and ginger. Cook as directed; sprinkle with 2 Tbsp. chopped toasted pecans before serving.
Curry variation: Add 1 tsp. curry powder with the onion and ginger; cook as directed.