Place the tomatillos and jalapeño in a medium saucepan with enough cold water to just cover (about 2-1/2 cups); bring to a boil.
Reduce heat and gently simmer about 8 minutes until tomatillos are tender.
Remove from heat; let stand for 15 minutes.
Place the tomatillos and jalapeño in blender along with cilantro leaves, garlic and cumin.
Blend for a few seconds, until smooth.
Over medium heat, heat the oil in the saucepan.
Add tomatillo mixture.
Bring to a simmer; reduce heat and cook until slightly thickened, 8 to 10 minutes.
Stir in salt and set aside to cool.
Before serving, fold in diced avocados, onion and chopped cilantro.
Serve with chips or fresh vegetables.
At this point, the sauce can be stored in the refrigerator for up to 3 days.
Before serving, reheat over low heat and loosen the sauce with a little water if necessary.
Cool before adding the avocados, onion and cilantro.