Combine the tomatillos, jalapenos, lettuce, garlic, cilantro, cumin, salt, and pepper in a blender or food processor; process until chopped to desired consistency.

Melt the butter in a saucepan over medium heat; cook the onions in the melted butter until clarified, about 5 minutes. Pour the tomatillo mixture and the chicken broth into the sauce pan; cook, stirring occasionally, until the salsa begins to thicken, about 20 minutes.