Preheat the oven to 400°F. Set the oysters on a cooling rack set on a baking sheet. Heat the butter in a medium pot over medium heat. Roast the oysters and cook the onions in the butter slowly. It important to cook the onions slowly and gently so they soften fully, but do not brown. You might want to cover the pot and cook them on low heat while you shuck the oysters.
If you are using pre-shucked oysters, or you just shuck raw oysters normally, just make sure the onions cook slow and at least 10 minutes, up to 20. Slow and low is the key here. Salt them as they cook.
While this is happening, strain the oyster liquor through a sieve with a paper towel set inside. Save this liquor, as it's the essence of the stew.
When the oysters and onions are ready, add the oysters and the liquor and the cream to the pot. You might need maybe a cup or two of water to fill things out, or you can add in extra oyster liquor, clam juice or fish stock here.
Season the stew with the pepper and the crab boil or a pinch of cayenne. Let this simmer 5 minutes, then stir in the chives and serve. Oyster crackers are a traditional garnish, and I highly recommend them if this is your main meal.
Notes
I love what the liquid crab boil brings to this stew, but it's not easy to find outside the South. Use a pinch of cayenne or Cajun seasoning instead if you can't get it.