Slice bread lengthwise in thirds. Toast bread lightly if desired. Soften Brie 1 minute in microwave, if necessary. With a fork or in food processor, crush or mash Brie cheese with butter and spices. Spread Brie butter over cut sides of bread, including both sides of middle slice. On bottom slice add half the onions, Swiss slices, pastrami, half the spinach, center slice of rye bread, remaining onions, Havarti and remaining spinach. Close sandwich with remaining Brie-buttered bread. Serve with Dijon mustard if desired.