Set a large bowl of ice water near the stove. Bring a medium pot of water to a boil. Add broccoli and onion; cook until the broccoli is bright green and tender-crisp, about 2 minutes. Drain; plunge the vegetables into the ice water. Let stand until chilled, about 2 minutes. Drain and pat very dry.
Whisk yogurt, mayonnaise, vinegar, honey, salt and pepper in a medium bowl. Fold in the broccoli mixture, grapes and almonds. Serve immediately.