Heat oven to broil. In medium bowl, combine lump crab meat, mayonnaise, red onion, celery, most of the chives, lemon juice, and a pinch of salt and pepper. Stir to combine. Evenly divide the mixture over the four English muffin halves and top each with a mound of shredded havarti. Place under the broiler for approximately 3 minutes until cheese melts and bubbles. Garnish each melt with the remaining chives. Serve immediately.