Preheat the oven to 150 c/300 f/gas mark 3.
Heat the milk and cream gently in a pan with the chopped lemon verbena.Remove from the heat and allow to cool before blending the mixture with a hand blender.
Place the egg yolks in a bowl,add the sugar and whisk together until pale and thickened.
Add the milk and cream mixture to the egg yolks and whisk well.Pass through a sieve and allow to rest for 10 minutes.Skim off the top layer to remove any foam.
Ladle the mixture into ramekins and place in a roasting tin.Place in the oven and pour in enough hot water to come halfways up the sides of the ramekins.Bake for 1.5 hours until set.Remove from the roasting tin and allow to cool.When cool,refrigerate the creme brulees for 2 hours,until firm.
To serve,drain the pears and cut into thin slices and arrange on the top of the ramekins.When ready to eat,sprinkle the demerara sugar over the top and caramelize either with a blowtorch or by putting them under a hot grill.