Toast the walnuts (optional but recommended): Warm a dry skillet over medium heat. Add the walnuts and cook for 3–4 minutes, stirring often, until fragrant and lightly golden. Set aside to cool.
Make the dressing: In a small bowl or jar, whisk together olive oil, apple cider vinegar, Dijon, honey, a pinch of salt, and a few grinds of pepper.
Taste and adjust—add more vinegar for tang, honey for sweetness, or salt to round it out.
Prep the apples: Core and thinly slice or dice the apples. Toss them with a squeeze of lemon juice to keep them crisp and bright.
Build the base: In a large bowl, add greens, sliced celery, and red onion. Drizzle with about half the dressing and toss gently so everything is lightly coated.
Add the stars: Fold in the apple slices and most of the walnuts.
If using, add dried cranberries and a sprinkle of goat cheese or feta.
Finish and serve: Add more dressing as needed and toss again. Top with the remaining walnuts and a final crack of pepper. Serve immediately.