Toast walnuts in a dry skillet over medium heat for 3-5 minutes until fragrant and lightly browned, stirring frequently. Set aside to cool.
Rinse cranberries and pulse in a food processor until coarsely chopped, or chop by hand. They should be in small pieces but not pureed.
In a small jar, combine olive oil, apple cider vinegar, honey, salt, black pepper, and cinnamon (if using). Seal tightly and shake vigorously until emulsified.
In a large bowl, combine mixed greens, chopped cranberries, toasted walnuts, crumbled feta, orange segments, and thinly sliced red onion.
Drizzle the dressing over the salad just before serving. Start with about three-quarters of the dressing, then add more if needed after tasting.
Toss gently to coat all ingredients evenly without bruising the greens.
For best flavor, let the dressed salad rest for 10-15 minutes in the refrigerator before serving.