Combine the mayonnaise and mustard in a small bowl and spread the mixture over 1 side of each slice of multigrain bread. Arrange the ham slices on 2 slices of bread, followed by the sliced turkey. Top with half of the Cheddar, then the tomato and the red onion. Next, top with the bacon and cornichon slices (if using), then the remainder of the cheese; sprinkle the chives over the filling before covering with the remaining 2 slices of bread.
Toast in a sandwich grill for 3 to 4 minutes, or according to the manufacturer’s instructions, until the filling is hot and melting and the bread is golden. Serve immediately with extra mayonnaise and mustard sauce and plenty of napkins!