8 cups fresh baby spinach 1/2 peeled avocado, cut into 8 thin wedges 1/4 cup crumbled feta
Cut the rind off the grapefruit. Holding the grapefruit over a bowl, cut between the membranes to release the fruit sections. Place the fruit sections in a fine sieve set over a bowl to catch the juices. You should have about 1/3+ cup juice at the end of this process. Whisk together the reserved juice, cilantro, yogurt, honey, and dijon. Season to taste with salt and freshly ground black pepper. Set aside.
Prepare a grill. Season the pork tenderloin on all sides with salt and freshly ground black pepper. Drizzle olive oil over top and turn to coat. Grill pork tenderloin for about 18 minutes, turning twice, or until the pork is cooked through. Remove the pork from the grill and transfer to a platter. Tent with foil to keep warm and let rest for 5 minutes. Slice the pork tenderloin into 1/2 inch pieces.
In a large bowl, toss the spinach with the reserved dressing. Divide the spinach among 4 salad plates. Top with grapefruit sections, avocado slices and pork tenderloin. Sprinkle feta over the top and serve.