To make ahead, prepare the greens, walnuts, and vinaigrette, then assemble the salad right before sitting down to dinner.
For raspberry vinaigrette, place oil, raspberry vinegar, mustard, sugar, salt, and pepper in a screw-top jar. Cover and shake to combine before using. Refrigerate up to 1 week.
Place greens and/or lettuces and sliced nectarine in a large bowl. Pour vinaigrette over salad; toss gently to coat. Sprinkle with toasted walnuts. Garnish with raspberries, if desired.