First, thinly slice both of the cheeses then set to one side.
Preheat a grill to high & set a large frying pan over a medium heat.
Next, take a slice of bread & top it with half of the sliced cheese. Make sure that the cheese completely covers the bread!
Spoon the caramelised onions on top of the cheese then top the onions with the remaining cheese slices.
Place the other slice of bread on top then spread a thin layer of butter over the top slice of bread.
Carefully place the toastie into the hot pan, butter side facing down then cook for 2-3 minutes, until golden brown. Press the toastie down occasionally with a spatula, so that the bread browns evenly.
Next, spread a thin layer of butter onto the top piece of bread then flip the toastie over & cook for another 2-3 minutes, until golden brown.
Once cooked, transfer the toastie to a baking tray then spread a generous amount of rarebit on top. Place the toastie under the grill then cook for another 2-3 minutes, until the rarebit is golden brown & bubbling.
To finish, use a knife to score the rarebit in a criss cross pattern then drizzle over some Worcestershire sauce then serve.