Lightly toast 2 slices of sourdough.
Place toast on a tray or pan that can go under the grill.
Warm the thermidor sauce in a pan.
Flake the crab in to the sauce and warm through stirring gently so the crab gets nicely covered.
Spoon the crab mixture on to the toast.
Place under a hot grill.
Remove once bubbling away and starting to colour.