1) Place a medium-sized skillet over medium heat. After about a minute, add the olive oil and swirl to coat the pan. 2) Add the onion and saute for 3-5 minutes, or until it begins to soften. Add the Brussels sprouts and salt, and saute for 5 minutes. 3) Sprinkle in 4 tablespoons water, shake the pan, and cover. Cook over medium heat for about 5-8 minutes, or until the Brussels sprouts are bright green and fork-tender. (You might need to add another tablespoon or two of water during this time to prevent sticking. Just keep your eye on it - and your fork intermittently in it.) 4) Using a small whisk in a medium-small bowl, beat together the mustard and maple syrup until smooth. Add this mixture to the pan and stir to combine. 5) Serve hot, warm, or at room temperature, topped with a shower of fresh black pepper, if desired.