Place onion & salt in a bowl large enough to hold the asparagus. Mix well & let steep for 30 minutes. Add the capers, lemon juice & mint leaf & marinate for at least 1 hour. Mix thoroughly from time to time. Meanwhile, bring an asparagus cooker to a boil & cook the asparagus until tender but still crisp, about 3 minutes. Drain. Toss the still warm asparagus with the marinade & add salt & pepper. Toss gently & serve.