Melt and Break Down the Onions
Melt the pork lard in a heavy-bottomed Dutch oven over medium-low heat. Add the finely diced onions and a pinch of salt. Cook slowly, stirring frequently, until completely soft and golden-translucent.
Slow cooking breaks down the cellular structure of the onions into a jam-like consistency. They will dissolve during the long simmer to naturally thicken the final broth without needing flour.
Onions should be golden-translucent, completely soft, and reduced significantly. Do not let them turn dark brown or crisp.