Place chilies in a 10- by 15-inch baking pan. Broil 2 to 3 inches from heat, turning occasionally, until charred all over, about 15 minutes. Let stand until cool enough to handle. Pull off and discard skins, stems, and seeds.
In a blender or food processor, whirl roasted chilies, lettuce, cilantro, tomatillos, green onions, and garlic until smooth. Pour the purée into a 1-quart glass measure and add enough broth to make 3 cups.
In a 2- to 3-quart pan over medium-high heat, frequently stir rice in oil until opaque, 3 to 4 minutes.
Stir in chili mixture and 1/2 teaspoon salt. Bring to a boil over high heat; cover, reduce heat, and simmer until rice is tender to bite, 18 to 20 minutes.
Stir with a fork, and add more salt to taste. Mound rice on a platter.