While your grill preheats, whisk together the avocado oil, apple cider vinegar, lime juice, honey, and minced garlic in a small bowl. Let this sit for 10 minutes so the flavors meld—this resting time is crucial for developing a balanced, cohesive dressing. Meanwhile, slice the red onion thinly, pit and slice the avocado into 1/4-inch wedges, and have your spinach, blueberries, and feta cheese ready. I like to prep these components just before grilling so the avocado doesn't oxidize and turn brown.
Pat the chicken breasts dry with paper towels, then rub them evenly on both sides with olive oil, kosher salt, and freshly ground black pepper. Heat your grill to medium-high heat (around 400°F if using a gas grill). Let it preheat for 5 minutes so it's hot enough to create good grill marks and cook the chicken through efficiently.
Place the seasoned chicken breasts on the grill and cook for 5 minutes without moving them—this allows a nice crust to form. Flip and cook another 5 minutes until the internal temperature reaches 165°F. During the last few minutes of chicken cooking, place the pineapple rings directly on the grill grates and cook for 2-3 minutes per side until they develop caramelized grill marks and soften slightly. I find that grilling the pineapple at the end means it won't overcook while waiting for the chicken to finish.
Transfer the chicken and pineapple to a cutting board and let rest for 5 minutes—this allows the chicken to retain its juices when sliced. Slice the chicken breasts into 1/2-inch thick strips and cut the pineapple rings into bite-sized chunks, discarding any tough outer pieces.
Place the fresh baby spinach in a large salad bowl or on individual plates. Arrange the sliced chicken, grilled pineapple chunks, blueberries, avocado slices, crumbled feta cheese, and red onion over the spinach. Sprinkle the fresh cilantro over the top.
Stir the dressing from Step 1 once more to recombine any settled ingredients. Drizzle the dressing over the assembled salad just before serving, tossing gently to coat all the components evenly without breaking up the avocado slices too much.