Cut the eel fillet diagonally into pieces that are 1 inch (2.5 cm) wide.
Toss the eel with a little salt and then the cornstarch.
Put the black beans, sugar, and 2 tablespoons of cold water into a small bowl and crush to a coarse paste.
Heat a wok over a high heat until it is smoking hot. Add the sesame oil and garlic, quickly followed by the ginger, red chile, and black bean paste. Stir-fry for a few seconds, then add the eel pieces and stir-fry for 1 minute.
Add the rice wine or sherry, soy sauce, and 3 to 4 tablespoons of water, and cook for 2 minutes, until the eel is cooked through.
Add the spring onions to the wok and stir-fry for about 1 minute. Serve immediately, with some steamed rice.