Grill or broil jalapeños until skin is dark green and blistering and you can peel them easily, about 5-10 minutes. Cool peppers, peel and remove seeds.
Grill or broil tomatoes about 5-10 minutes until you can peel skin.
Place peppers and tomatoes in blender and process just until chunky. Pour into bowl; add onion and cilantro, mix well. Add salt to taste.