Cut avocado in half, remove seed and skin, cut flesh into slices.
Lay out 4 slices of bread spread on mustard, place beef and avocado slices on top and cover with remaining bread.
Spread butter over the top of the bread, turn over and place buttered side onto a sandwich grill, spread butter onto the remaining side, close the grill and cook until golden.
Cut sandwich diagonally in half and serve.
Serve with a fresh rocket, tomato and olive salad.
Serving Size 4