Melt butter in a heavy saucepan over medium-high heat; add onions, and sauté 3 minutes or until tender. Whisk in flour, and cook, whisking constantly, 2 minutes. Add cream, and cook, whisking constantly, until smooth and sauce begins to bubble. Remove from heat, and stir in cheese until smooth. Stir in sherry, salt, and pepper. Gently fold in crabmeat and parsley. Keep warm in a chafing dish or slow cooker set on WARM or LOW. Serve with toast points.