10 ounces fresh baby spinach, stems removed
1/2 medium red onion, thinly sliced
2 tablespoons olive oil
8 ounces white mushrooms, sliced
Kosher salt
6 ounces thinly sliced pancetta, cut into 1 1/2-inch strips
1/4 cup red wine vinegar
1 tablespoon honey
1/2 teaspoon Dijon mustard
Freshly ground black pepper
2 ounces pine nuts, toasted (about 1/3 cup)