Halve one avocado, remove pit, and scoop flesh into medium bowl. Mash flesh lightly with onion, garlic, jalapeño, cilantro, salt, and cumin (if using) with tines of a fork until just combined.
Halve and pit remaining two avocados, and prepare according to illustrations 1 and 2 below. Gently scoop out avocado into bowl with mashed avocado mixture.
Sprinkle lime juice over diced avocado and mix entire contents of bowl lightly with fork until combined but still chunky. Adjust seasoning with salt, if necessary, and serve. (Can be covered with plastic wrap, pressed directly onto surface of mixture, and refrigerated up to one day. Return guacamole to room temperature, removing plastic wrap at the last moment, before serving).
GUACAMOLE WITH BACON, SCALLIONS, AND TOMATO (Retired cooking teacher and Cook's friend Bernice Sisson inspired this variation.)
Follow recipe for Chunky Guacamole, substituting 3 large scallions, sliced thin (about 1/3 cup), for onion and adding 6 slices cooked, drained, and crumbled bacon with 1 teaspoon rendered fat and half a medium tomato, seeded and diced small.