Brush poblano, bell pepper, romaine heart, and green onions with remaining 1 tablespoon oil. For a charcoal grill or gas grill, place poblano, bell pepper, and green onions on the greased rack directly over medium heat; cover and grill until tender (about 10 minutes for poblano and bell pepper, turning once, and about 1 minute for green onions). Cool; then thinly slice all the vegetables. Place romaine and lime halves, cut side down, on the grate and grill 2 minutes, or until grill marks form. Cool; then chop romaine. Cut lime halves into wedges.