Gather squash, sweet onions, and jalapeno pepper.

Add bacon grease and butter to a large cast iron skillet.  There's no need to heat the skillet prior.

Trim and thinly  slice squash into rounds of equal thickness. This looks like a pile of squash, and it is. Squash contains a lot of water and it will cook down significantly.

Cut onions in half and thinly slice into rounds.

Dice jalapeno pepper.  If you don't wear gloves, be sure to wash your hands afterwards.

Add vegetables and seasoning to skillet containing bacon grease and butter.

Cover and place over medium heat.  No need to add water.  Squash contains a good amount of water. You want the vegetables to stew in their own juices. Cook for about 10 minutes or until squash is tender.

Remove cover. Cook until most of the juices have evaporated. Stir occasionally.

Continue cooking until the vegetables reach your desired level of caramelization. Brown means flavor. Taste for seasoning. Serve warm.