Clean your fish and pat dry with a kitchen towel. With a sharp knife, make a slit in the belly almost to the tail.
Mix together black bean, garlic, ginger, and half the scallions; stuff the mixture into the slit. Place the fish in a medium-sized plate and pour over with vegetable and sesame oil. Squeeze a bit of lemon juice over the fish.
Fill your wok or skillet with 2 to 3 inches of water and bring to boil. Carefully set your plate on the rack, then cover with the lid. Steam for 10 to 12 minutes, depending on how thick your fish is in the middle. Check for doneness by poking the flesh with a spoon or chopstick at the thickest point; if the flesh flakes off easily, your fish is done.
Garnish with the other half of your scallions and serve immediately.