Make salsa dressing: in a bowl, stir together 2 parts salsa to 1 part crema. Thin with water to a dressing consistency. Season to taste with salt, and chill.
Prepare a charcoal or gas grill for high heat (450° to 550°; you can hold your hand 5 in. above cooking grate only 2 to 4 seconds).
Meanwhile, finely chop lettuce and arrange on a platter. Season steak and onion to taste with oil, salt, and pepper.
Grill steak and onions, turning often, until steak is done the way you like and onion is lightly charred.
Cut steak into thin strips and mix with enough dressing to coat. Arrange onions and steak over lettuce. Scatter cilantro and pepitas on top. Serve with more dressing on the side.
*Buy from a Mexican market.