Balsamic Vinegar Sauce

• 8 ounces unsalted butter
• 1 medium red onion, peeled and thinly sliced
• 2 cloves garlic, peeled and thinly sliced
• 8 shiitake mushroom stems
• 1 medium ripe tomato, coarsely chopped
• 1 bay leaf
• 1 teaspoon black peppercorns
• 13 cup balsamic vinegar
• 13 cup water
• Salt and freshly ground black pepper to taste

1. Cut 6 tablespoons of the butter into 1/2-inch cubes.
2. Refrigerate.
3. Melt the remaining butter in a large saucepan over medium heat.
4. Add onion, garlic, mushroom stems, tomato and bay leaf.
5. Cook, stirring occasionally, until the vegetables are soft but not brown, about 6 minutes.
6. Add vinegar and water.
7. Reduce over medium high heat until just a syrupy liquid remains, about 8 minutes.
8. Lower the heat slightly.
9. Whisk in the cold butter, 2 to 3 pieces at a time, allowing each addition to be absorbed before adding the next.
10. Strain through a fine sieve.
11. Season with salt and pepper.
12. Keep warm over a double boiler.

butter, red onion, garlic, shiitake, tomato, bay leaf, black peppercorns, balsamic vinegar, water, salt

Taken from cooking.nytimes.com/recipes/9093 (may not work)