Combine the onion, rice vinegar, and sea salt together in a small mixing bowl. Allow the onions to marinate at room temperature for 20-30 minutes, tossing occasionally.
Meanwhile, heat a small, dry skillet over medium heat and add the pine nuts. Toast, stirring often, until they are fragrant and golden brown, about 8-10 minutes.
Divide the spinach and pears between four plates. Add the onions, reserving the vinegar. Top the salads with the blue cheese and toasted pine nuts.
Whisk the olive oil together with the reserved vinegar and black pepper. Spoon the dressing over the salads to taste.