Wipe chili with a damp cloth. Using scissors, cut off and discard chili stem end; remove and discard seeds. Cut chili crosswise into 1/4-inch strips.
In a 10- to 12-inch nonstick frying pan over medium-low heat, stir chili in oil until it smells toasted and feels slightly crisp (avoid scorching), 1 to 2 minutes. With a slotted spoon, transfer chili pieces to towels to drain (they crisp as they cool).
Add onion and garlic to pan. Stir often over medium-high heat until onion is lightly browned, about 5 minutes.
Add amaranth and quinoa. Stir until grains are slightly toasted, about 1 minute. Mix in corn and broth. Bring to a boil over high heat; cover and simmer over medium heat until liquid is absorbed, about 15 minutes.
Let cool to room temperature. Or nest pan in ice water and stir occasionally with a fork until cool, about 10 minutes.
Mix lime juice and cilantro with grains. Spoon onto plates lined with lettuce leaves. Top salads with the crisp chili pieces. Add salt and pepper to taste.