1-1/2 pounds green beans, trimmed
2-1/2 ounces thinly sliced pancetta (five or six 1/4- to 1/2-inch-thick slices)
3 tablespoons extra virgin olive oil, divided
6 medium cremini mushrooms, trimmed, halved if large, and very thinly sliced
2 medium-large shallots, halved lengthwise and very thinly sliced
1/4 cup very thinly sliced fresh sage leaves
1 tablespoon sherry vinegar
1/2 teaspoon Dijon mustard
Fill a large mixing bowl with ice cubes and water and set aside. Fill a 6- or 7-quart pot two-thirds full of well-salted water. Bring the water to a boil, and boil the beans, uncovered, until tender to the bite, 4 to 6 minutes. Drain, transfer the beans to the bowl of ice water, and let sit until cooled, about 2 minutes. (This completely stops the cooking process and retains the color of the beans.) Drain and pat dry.
Put the pancetta in a 12-inch nonstick skillet and cook over medium-low heat until crisp and browned, 10 to 12 minutes. Transfer to a paper towel-lined plate and coarsely crumble. Remove the pan from the heat and let it cool slightly.
Return the pan to medium heat, add the green beans and toss to combine and heat through, 2 to 3 minutes. Season to taste with salt. Transfer to a warm serving platter and garnish with the pancetta.
Make ahead: The beans can be boiled and refrigerated up to 6 hours ahead. The remaining ingredients can also be prepped up to 6 hours ahead and held in the refrigerator. An hour before finishing, remove the beans from the refrigerator to come to room temperature.
Source: Fine Cooking, Oct/Nov 2009