Heat oil in a fry pan, add onion and cook until soft, then remove from pan and set aside.
Place fry pan back onto the heat, season steaks and cook to your liking.
Cut avocado in half, remove seed and skin, cut flesh into slices.
Lay out four slices of bread and top with steak, onion, avocado, tomato chutney and remaining slices of 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.
Serving Size 4