In a large, shallow pan big enough to fit a bamboo steamer basket, combine vegetable stock, lemongrass, and mushrooms; bring to a simmer.
Meanwhile, using a sharp knife, make 3 to 5 slits on each side of fish, at an angle and about 1 1/2 inches apart. Place ginger inside each slit.
Line bottom section of a bamboo steamer with cabbage leaves and lay fish on top of leaves. Cover with half the cilantro and 1/4 cup green onions, and place steamer lid on top. Place steamer over simmering water and cook until fish is cooked through and flesh flakes easily, about 20 minutes. Remove fish and steamer from pot, and place fish on a serving platter. Set aside.
To simmering liquid, add beer, remaining cilantro, and 1/4 cup green onions and bring to a boil. Reduce to a simmer and cook until slightly reduced, about 5 minutes.
To serve, pour sauce over fish and garnish with cilantro and green onions.