In a mixing bowl, beat cream cheese and butter. Add cheese, onions and garlic salt; mix well. Spread on both sides of each slice of bread.
Wrap loaf in a large piece of heavy-duty foil (about 28-by-18-inches); seal tightly. Grill, covered, over medium heat for 8–10 minutes, turning once. Unwrap foil; grill 5 minutes longer.