4 boneless skinless chicken breast halves (6 to 8 ounces each)
Heat 1 tablespoon oil in a medium saucepan over medium-low heat. Add onion, garlic, and jalapeno; cook, stirring occasionally, until softened, 5 to 8 minutes.
Add pumpkin seeds; cook, stirring often, until puffed, about 5 minutes. Add lettuce, vinegar, and 1 cup water; season with 1? teaspoons salt. Bring to a boil; reduce heat, and simmer, covered, until lettuce is tender, about 10 minutes.
Transfer mixture to a blender; add cilantro. Blend on high speed until sauce is very smooth, about 3 minutes. Thin sauce with 2 to 4 tablespoons more water, if needed (it should be pourable).
Meanwhile, heat grill to medium high; oil grates. Rub chicken with remaining tablespoon oil; season generously with salt and pepper. Grill, turning once, until browned and cooked through, 6 to 8 minutes per side. Serve with sauce.