Soak onion slices in cold water 10 minutes; drain well. While onion soaks, whisk together kefir, mayonnaise, olive oil, lemon rind, pepper, salt, and garlic in a large bowl. Coarsely chop broccoli florets. Peel stalks; cut stalks into half-moons. Add broccoli, onion, and sauerkraut to kefir mixture; toss to coat. Sprinkle with almonds.