Bring a large pot of salted water to boil. Coarsely chop the broccoli florets and tender stems. Add to the water and cook until crisp tender, about 2 minutes. Drain well, pressing on the broccoli to release any excess water.
Whisk the buttermilk, mayonnaise, parsley and lemon zest together in a medium bowl until smooth; season with salt and pepper. Add the broccoli and toss well. Top with the sunflower seeds.
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