To make the filling, mix together all the ingredients (except the gelatine, nutmeg and salt and pepper) in a bowl using a hand blender.
Pour into a saucepan over a medium heat and slowly bring to the boil while constantly whisking for 2–3 minutes.
Transfer to a thermomix or blender and blitz with the gelatine until smooth and emulsified. Season with salt, pepper and nutmeg. Pass through a fine sieve, then pour into a piping bag and place in the fridge until set.
To make the balsamic jelly, dissolve the sugar in the balsamic vinegar in a saucepan and warm gently over a low heat. Add the gelatine and stir until dissolved. Pass through a fine sieve, pour into a piping bag and leave to set in the fridge.
Put all the pesto ingredients into a blender or small food processor and blitz together.
To assemble, pipe the filling into the tart cases and top with a layer of pesto, then scatter over the tomatoes and basil leaves. Finish with a few dots of balsamic jelly.