Peel and thinly slice the red onion into half-moons. If you prefer a milder onion flavor, place the sliced onion in a small bowl of cold water for 10 minutes, then drain and pat dry.

Toast walnuts

Place walnuts in a dry skillet over medium heat. Toast, stirring or shaking the pan, until fragrant and slightly darkened at the edges, about 3–4 minutes. Remove from heat and let cool slightly, then roughly chop.

Assemble salad

In a large salad bowl, combine the mixed salad greens, drained/sliced red onion, dried cranberries, and toasted walnuts. If using, sprinkle the crumbled feta or goat cheese on top.

Dressing

Make vinaigrette

In a small bowl or jar, add the red wine vinegar, Dijon mustard, honey, salt, and pepper. Whisk briefly to combine. Slowly drizzle in the olive oil while whisking (or close the jar and shake vigorously) until the dressing is emulsified and slightly thickened, about 30 seconds.

Dress and toss

Right before serving, drizzle about half to two-thirds of the vinaigrette over the salad (start with less; you can add more to taste). Gently toss the salad until ingredients are evenly coated. Taste and add more dressing, salt, or pepper as needed. Serve immediately.