Add avocados, onions, and jalapeno to bowl. Mash to desired consistency.
Stir in tomato, cilantro, salt, and cumin. Sprinkle with lime juice and stir gently.
Serve immediately, or if you prefer to make ahead of time, smooth the top of the guacamole with a spoon and cover with a layer of plastic wrap so that the wrap is directly on top of the guacamole. This will aide in keeping it fresh and the avocado from turning brown. Refrigerate.