1. In a small skillet, toast the cumin seeds over moderately high heat, shaking the skillet, until fragrant, about 30 seconds. Transfer the cumin seeds to a mortar to cool completely, then finely grind with a pestle.
2. Scoop half of the avocados into a large bowl and coarsely mash them with a fork. Stir in the lemon juice, cilantro, onion, garlic, jalapeño and ground cumin. Scoop chunks of the remaining avocados into the bowl; stir lightly with a spoon. Season the guacamole with salt and serve at room temperature or lightly chilled.