1. Trim outer leaves and stems from brussels sprouts, and discard. Bring a medium pot of water to a boil, and add salt. Meanwhile, prepare an ice-water bath. Add brussels sprouts to boiling water, and cook until tender but still bright green, about 4 minutes. Remove from heat, drain, and plunge into ice-water bath to cool. Drain well, and cut in half.
2. Heat 1/2 tablespoon butter and 1/2 tablespoon olive oil in a large heavy skillet over medium-high heat. Add Brussels sprouts, and cook, tossing occasionally, until they are brown and crisp on the edges, about 3 minutes. Season to taste with salt and pepper, and transfer to a large bowl. Cover with aluminum foil to keep warm.
3. Add remaining 1/2 tablespoon each butter and oil to the same pan over medium-low heat. Add onions, and cook, tossing occasionally, until wilted and transparent, about 3 to 4 minutes. Add vinegar (stand back to avoid the fumes), and stir to loosen any brown bits on bottom of pan. Cook until vinegar is reduced and the onions are glazed, about 30 seconds.
4. Add onions to Brussels sprouts, and toss well. Serve immediately.

Yield: 4 servings