Green Beans with Carmelized Red Onion and Mushroom

2 tablespoons olive oil
1/2 pound cremini mushrooms, coarsely chopped
2 tablespoons unsalted butter
1 red onion, finely chopped
1 teaspoon balsamic vinegar
1 garlic clove, minced
2 tablespoons finely chopped fresh parsley
Salt and freshly ground black pepper
2 pounds tender green beans, ends removed

1. In a medium skillet, heat 1 tablespoon of the oil over medium heat. Sauté the mushrooms for 3 to 4 minutes, or until they change color and soften. Remove to a bowl.

2. Add the remaining 1 tablespoon oil and 1 tablespoon of the butter. Sauté the onion for 7 to 10 minutes, or until it is soft and begins to caramelize. Add the vinegar and cook, stirring constantly, for 1 minute. Add the garlic and sauté for 1 minute. Add the garlic and sauté a for 1 minute. Add the mushrooms, parsley, and salt and pepper to taste. Remove from the heat.

3. Bring a large saucepan of water to a boil. Salt the water, immerse the beans in the boiling water, and cook for 5 to 7 minutes, or until tender but slightly resistant. Drain and place in a serving dish. Toss with the remaining 1 tablespoon butter and salt and pepper to taste. Sprinkle evenly with the topping and serve immediately.

ADVANCE PREPARATION:
Make 1 day ahead through step 1, cover, and refrigerate. Reheat the topping in a skillet before serving.