Place the sliced red onion in a bowl of cold water and let it sit for 5–10 minutes. Drain well and pat dry with a paper towel.
Heat a dry skillet over medium heat. Add walnuts and toast, stirring often, until fragrant and lightly browned, about 3–4 minutes. Remove from heat and let cool.
In a large serving bowl, combine the spinach, blueberries, feta, drained onions, and toasted walnuts.
In a small bowl or jar with a lid, whisk or shake together the balsamic vinegar, maple syrup, Dijon mustard, olive oil, salt, and pepper until well combined and emulsified.
Drizzle the dressing over the salad right before serving and toss gently.
Top with avocado slices if using, and serve immediately.
Notes
Make-ahead tip: Combine salad ingredients (except avocado) and refrigerate. Make dressing and store it separately. Dress and add avocado just before serving.
Greens swap: Arugula, mixed baby greens, or massaged kale also work well here.