Blanch the french beans in boiling water for 2 minutes, then drain and refresh in ice-cold water (to stop it from cooking further).
Put the peanut butter, soy sauce, rice vinegar, brown sugar in a bowl and mix together:
Add 1 to 2 tbsp of hot water to loosen the mixture if it is very thick. Stir until cooled and sugar is dissolved.
Place a heavy frying pan or wok over medium heat and add a dash of oil.
Add the finely chopped garlic and cook for 1 minute until golden. Toss in the beans and cook for 2-3 minutes until beginning to blister slightly on the outside.
Lower the heat and add the peanut dressing. Stir well.
Transfer the beans to a serving bowl and sprinkle with toasted sesame seeds.