Warm Spinach and Frisée Salad with Sweet Onion

3 tablespoons plus 1 teaspoon white wine vinegar
1 large (12 oz.) sweet onion, sliced thin
4 ounces pancetta or thick-sliced bacon, diced
4 ounces shiitake or white mushrooms, sliced
1/4 teaspoon salt
1/8 teaspoon pepper
2 tablespoons olive oil
1 bag (10 oz.) fresh spinach, trimmed
1 head frisée or chicoty lettuce (4 oz.)

1. Bring 1 1/2 quarts water to boil in medium saucepan. Remove from heat; stir in 1teaspoon vinegar and half the onion slices (reserve remaining onion). Let stand one minute; remove onions and drain on paper towels.
2. Dressing: Heat skillet 1 minute over medium heat. Add pancetta and cook until crisp. Stir in remaining onion and mushrooms; cook until softened. Stir in remaining vinegar, salt and pepper. Remove from heat; add olive oil.
3. Toss spinach, frisée and onion in large bowl. Toss with warm dressing.