There are a couple of ways to do this well.  Modify this recipe based on what you have available.
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• In a Dutch Oven or large pan with lid, add 2 T of butter and 1 sliced onion.  Cook on low heat until onions are soft.
  • If you don't have an onion you can skip this step but even if you aren't going to eat the onions, this step adds good flavor.
• Nestle the brats in the onions, and add one bottle of beer.
  • Alternately you can use 1.5 cups of apple juice, apple cider, or chicken stock.
• Bring the a boil and then reduce heat and cover.  Cook for 20-30 min, depending on how many brats you have.
  • You can also do all of this in an instant pot and then just cook for 8 min on high pressure.
• Take everything out of the pan, add a little more butter if needed, add back the brats only and cook until browned.
  • You can also grill them at this stage.  
• Serve with or without the onions.  With or without mustard.  With or without a bun.  Whatever sounds good to you!