Spice-Rubbed Pork Tenderloin with Pineapple and Spinach Salad

Heat grill to medium-high. In a small bowl, combine the brown sugar, chili powder, cumin, 2 teaspoons oil, and 1/2 teaspoon each salt and pepper. Rub the mixture all over the pork and grill, covered, turning occasionally, until the internal temperature reads 145 degrees F, 16 to 20 minutes. Transfer to a cutting board and let rest for at least 3 minutes before slicing. Meanwhile, in a large bowl, toss together the pineapple and onion with the remaining tablespoon oil. Place on the grill with the pork and cook until the pineapple is lightly charred and the onions are tender, 3 to 4 minutes per side; transfer to a cutting board. Cut the pineapple into big chunks, separate the onion rings, and return to the large bowl. Add the spinach, season with 1/4 teaspoon pepper, and gently toss until the spinach begins to wilt, about 2 minutes. Serve with the pork.