Thinly slice the onion and cut the bacon into thin strips.

Heat a pot over low heat and melt the butter. Add the bacon and onion, and sauté for about 3 minutes without letting them burn.

Once the onion becomes translucent, add the flour and stir until the flour is fully incorporated and no longer powdery.

Add water and salt, bring to a boil, then cover and simmer on low heat for 7–8 minutes.

Add the milk and gently heat on low until the soup thickens.