In a small bowl or jar, combine olive oil, balsamic vinegar, honey, and Dijon mustard. Whisk or shake until the dressing becomes smooth and well blended. Add salt and pepper, then taste and adjust sweetness or acidity as needed.
Place chopped walnuts in a dry skillet over medium heat. Toast them for about 3–5 minutes, stirring frequently until fragrant. Remove immediately from heat to prevent burning and let them cool completely.
Rinse and dry the arugula thoroughly so the dressing sticks properly. Slice the avocado and red onion thinly for balanced flavor in every bite. Gently pat the blackberries dry after washing.
Add arugula to a large mixing bowl as the base. Top with blackberries, avocado slices, red onion, toasted walnuts, and crumbled feta. Distribute ingredients evenly for a colorful presentation.
Drizzle the dressing over the salad just before serving. Toss gently using salad tongs to avoid smashing the avocado and berries. Serve immediately for the freshest taste and texture.