Finely slice the finger chili.
Peel and finely slice the garlic cloves and shallot.
Place the sliced chili, garlic, shallot and sugar in a small jar with a screw-type cap. Pour in the lime (or lemon) juice, fish sauce and oyster sauce. Screw on the jar cap and shake the jar until the sugar is dissolved.
Start boiling water in your steamer.
Pat the fish steaks dry with paper towels. Arrange in a shallow heat-proof bowl. Pour the sauce over and around the fish steaks.
Place the bowl in the steamer basket. Steam the fish steaks over boiling water for 15 to 20 minutes (depending on their size).
Off the heat, sprinkle the Vietnamese-style steamed fish steaks with snipped scallions before serving.