For the dressing:
Sweet-tart berries are the perfect foil for rich avocado and creamy, fresh goat cheese. Add soft curls of butter lettuce, walnuts, and red onion, and you have a gorgeous salad.
In a medium-size bowl, whisk together vinegar, shallot, mustard, salt, and pepper. Drizzle olive oil in a thin stream, whisking constantly, until an emulsion forms. Set aside.
For the salad:
In a heavy-bottomed skillet over medium-low heat, toast walnuts, stirring often, until lightly browned and fragrant, about 10 minutes. Set aside.
Toss lettuce with enough vinaigrette to coat leaves lightly. Arrange on 6 plates. Gently toss berries, avocado slices, and onion in more vinaigrette and divide among plates. Top with walnuts and chèvre.