Mix mint, lemon peel, garlic, peas, and lettuce; season to taste with salt and pepper.
Pat mixture gently into a neat mound on an 8- to 10-inch rimmed plate at least 1/2 inch smaller in diameter than your steamer.
Rinse trout and pat dry.
Then neatly arrange fish over lettuce, cavity sides down and heads pointing in same direction; lean fish against each other.
Lay lemon wedges on top.
Place a steamer rack over about 1 inch boiling water in a steamer or large pan.
Carefully place plate on rack.
Cover and steam until fish is just opaque but still moist in thickest part; cut to test about 15 minutes.
If necessary, add boiling water to keep at least 1/2 inch of water in steamer.
Using thick pot holders, lift plate from steamer.
Transfer trout to warm individual plates and spoon lettuce mixture alongside.
Sprinkle trout with lemon juice.