1. In a large bowl, whisk together mustard, lime juice, and oil; season with salt and pepper. Add onion, bell pepper, and mango; toss to combine. Set aside.

2. Lay tortillas on a work surface; dividing evenly, line each with spinach, leaving a 1-inch border all around. Arrange beef slices down center of each wrap. Top beef with mango mixture and any juices from bowl.

3. Fold in 2 opposite sides, and roll up wraps tightly; cut in half.

4. For Black Bean Relish: In a medium bowl, combine 1/2 medium red onion, finely chopped, one can (15.5 ounces) black beans, rinsed and drained 1 cup chopped fresh cilantro, 2 tablespoons fresh lime juice, and 1 tablespoon olive oil. Season with salt and pepper.