Melt the butter in a large saucepan over low heat.
Add onion and garlic and stir well.
Let cook gently, without colorings, until the onion is soft and translucent.
Stir in the flour and cook, stirring constantly, 1-2 minutes.
Do not allow the flour to brown.
Add the chicken stock, a little at a time, and stir until the flour thoroughly mixed and liquid is smooth and free of lumps.
Bring to a boil and reduce the heat.
Cut the avocados in half and remove the thick green skin and stones.
Place the meat in a bowl and mash well with a fork.
Add crab meat and mix together.
Add the mashed avocado and crab in the soup and simmer gently for 15-20 minutes cooking.
Stir in the cream and season to taste with salt and pepper.
Remove from heat and set aside to cool.
Cover and chill in the refrigerator.
Serve chilled, piece topped with avocado and chives or cilantro.