A double boiler isn't necessary if you keep the heat low enough so that it doesn't go above a simmer but most people are too impatient for that.
Here's one of my favorite oyster stew recipes for when I want a change from the basic.
OYSTERS ROCKEFELLER STEW – 4 servings
4 slices bacon
1 cup thinly sliced leeks
1 cup thinly sliced celery
1/ 8 tsp. cayenne pepper
3 cups half-and-half
2 cups milk
1 1/ 2 pints shucked oysters and liquor
2 tsp. Worcestershire sauce
2 cups thinly sliced raw spinach leaves
Salt and black pepper
1 Tbs. butter (optional)
Oyster crackers to serve with stew
In a large heavy saucepan or soup kettle, fry the bacon until crisp. Drain on paper towel, crumble and reserve. Pour off all but 2 Tbs. of drippings from the skillet.
Add the leek and celery and saute over medium-low heat for 8-10 minutes, until the vegetables are softened. Stir in the cayenne pepper.
Add the half-and-half and the milk and bring to a simmer. Add the oysters (cut large ones in half) and the Worcestershire sauce. Return to a simmer. Stir in the spinach and simmer for about 5 minutes, until the spinach is just wilted and the oysters are cooked through. Add salt and pepper to taste.
Swirl in the optional butter, if used, and serve immediately in large, shallow bowls. Sprinkle each bowl with the reserved bacon.