Make the vinaigrette. Whisk all of the vinaigrette ingredients together in a mixing cup (or shake in a mason jar) until emulsified.
Assemble the salad. Combine the greens, strawberries, blueberries, pecans, feta and onion in a large mixing bowl. Drizzle evenly with the vinaigrette and toss gently to combine.
Serve. Serve immediately, garnished with extra feta and a twist of freshly-ground black pepper if desired, and enjoy!