Spinach Salad and Pancetta Vinaigrette

1/4 cup minced pancetta (Italian bacon)
1 shallot, minced
2 tablespoons sherry vinegar
2 tablespoons balsamic vinegar
1 tablespoon red-wine vinegar
1 teaspoon Dijon mustard
2 teaspoons chopped fresh thyme
1/3 to 1/2 cup olive oil
salt and pepper to taste
6 cups baby spinach

In a saucepan over medium heat, cook pancetta until golden brown but not crisp. Pour off grease, strain, and reserve. Lightly sauté shallot in same pan, but do not brown. Add vinegars and reduce volume by half. Remove from stove and stir in Dijon mustard and thyme, then whisk in olive oil and reserved warm grease from pancetta. Season with salt and pepper. Just before serving, toss spinach with dressing (rewarmed and rewhisked if necessary).