In a bowl or jar, combine oil, vinegar, sugar or honey, poppy seeds, mustard, salt, and pepper
Shake or whisk until the dressing is well mixed and slightly thickened
Refrigerate the dressing for at least 10 minutes before using
Place fresh spinach in a large salad bowl
Add sliced strawberries, blueberries, raspberries, red onion, feta cheese, and almonds
Drizzle the chilled poppyseed dressing over the salad just before serving
Toss gently to combine without crushing the berries
Serve immediately for best texture and flavor