Scrub and de-beard the mussels, then set aside.
In a wok over high heat, add the chicken stock (or water) and mussels, cover with a lid and cook for 5 minutes or until the mussels open. Strain the mussels, reserving the cooking liquid for later use.
Put the wok back over medium heat, add the oil and gently fry the garlic and chilli. Once golden, add the mussels and increase the heat. Toss through the oyster sauce, fish sauce and Asian basil, then add 4 ounces/1/2 cup of the reserved cooking liquid. Mix the potato starch with the water and toss it through the mussels to thicken the sauce. Season the mussels with sugar and pepper and garnish with more sliced chilli and fresh basil.