1. In a covered 5- to 6-quart pan over high heat, bring potatoes, broth, ginger, and lemon peel to a boil. Reduce heat to low and simmer until potatoes are almost tender when pierced, 12 to 15 minutes.
2. Meanwhile, rinse fish and pat dry; cut into 4 equal pieces. Rub fish all over with olive oil and place pieces, skin down and slightly apart, in a 10- by 15-inch baking pan.
3. Bake in a 300° regular or convection oven until fish is barely opaque but still moist-looking in center of thickest part (cut to test), 14 to 16 minutes.
4. Rinse cabbage and cut lengthwise into quarters. When potatoes are almost tender, set cabbage on top of potatoes. Cover, turn heat to medium, and steam until cabbage is tender when pierced, about 5 minutes. With a slotted spoon, lift out cabbage and potatoes and arrange equally in 4 wide bowls. Slide a wide spatula between fish and skin and lift fillet pieces off skin; arrange 1 piece of fish alongside vegetables in each bowl.
5. With a slotted spoon, remove and discard lemon peel and ginger from broth. Stir in peas and bring to a boil over high heat. Ladle broth and peas equally over fish and vegetables. Offer lemon wedges to squeeze over portions. Add salt and pepper to taste.