Prepare one batch of my Apple Cider Vinegar Vinaigrette and set aside.
Place the sliced red onion in a small bowl with 2 to 3 tablespoons of the vinaigrette and let it sit for 10 minutes while you prepare the rest of the salad (optional; see Cook's Notes).
Toast the walnuts in a dry skillet over medium heat for 2 to 3 minutes, stirring frequently, until fragrant. Let cool slightly, then roughly chop. (See Cook's Notes.)
Add the spinach to a large serving bowl or platter.
Pour some of the vinaigrette over the greens and toss gently so every leaf is lightly coated.
Spoon the marinated onions over the spinach. Top with the apples, dried cranberries, toasted walnuts, sunflower seeds, and goat cheese.
Drizzle with the remaining vinaigrette just before serving and toss gently, or serve the vinaigrette on the side.