Beef and Mango Wraps with Black Bean Relish

Name: Beef and Mango Wraps with Black Bean Relish
Ingredients: 1 tablespoon Dijon mustard; 3 tablespoons fresh lime juice from 2 limes; 1 tablespoon olive oil; Coarse salt and ground pepper; 1/2 medium red onion thinly sliced; 1 red bell pepper (ribs and seeds removed) halved lengthwise and cut crosswise into thin strips; 1 mango (about 1 pound) peeled and cut into 1/2-inch-wide strips; 4 whole-wheat tortillas (10-inch); 4 cups loose spinach (about 5 ounces) trimmed, well washed and dried; 1 lb. cooked beef very thinly sliced, room temperature; Black-Bean Relish (optional)