Cut four 3cm-thick slices from loaf. Cut a pocket lengthways through the centre of each slice of bread (do not cut all the way through). Spread inside of pockets with mustard. Fill with cheese and ham.
Whisk eggs and milk in a shallow dish. Season with salt and pepper.
Heat a non-stick frying pan or barbecue plate over medium heat. Add half the butter and 1 tablespoon oil. Brush surface to coat. Dip bread, 1 slice at a time, into egg mixture and turn to coat. Hold over dish to drain. Cook, in batches, for 3 minutes each side or until golden, adding remaining butter and oil as required. Slice French toast in half crossways. Serve.