Bring 6 cups (1.5L) salted water to a boil in a large saucepan.
While water is heating, prepare an ice bath in a large bowl.
Add broccoli to boiling water and cook for 1 minute.
Drain broccoli immediately and plunge into the ice bath to stop cooking.
Once cooled, drain broccoli well and spin in a salad spinner at least twice to remove excess moisture. Alternatively, pat dry with clean towels.
In a medium bowl, whisk together sour cream, vinegar, and mustard. Season to taste with kosher salt and black pepper
Place broccoli in a large serving bowl and add feta, red onion, spring onions, almonds, chives, and dill.
Pour dressing over salad ingredients and toss until evenly coated.
Chill for at least 30 minutes for best flavor, or serve immediately.