1. In a nonreactive saucepan (stainless, 4-quart) over moderately low heat, melt 3 tbsp of butter. Add onion and salt; stir well to coat. Cook slowly, stirring often, until the onion is tender but not browned, about 10 minutes. Sprinkle flour over onion, stirring well to blend, and cook 2 minutes longer. Slowly whisk in heated milk and heavy cream, along with reserved oyster liquor. Bring just to a light simmer, stirring often to prevent scorching.
2. In a wide nonstick skillet over medium-high heat, melt remaining 2 tbsp of butter until hot and bubbling, but not browned. Add drained oysters in a single layer. Sprinkle with coarse salt and black pepper; sauté until oysters begin to curl around edges and gills are slightly exposed, about 2 minutes. Do not overcook. Transfer entire contents of oyster sauté pan to the onion-milk mixture. Stir in a pinch of cayenne, cover, and remove from heat to "ripen" for 10 minutes, allowing flavors to develop.
3. While soup is resting, use cookie cutter (or open end of a drinking glass) to cut a 3-inch round from each slice of bread. Butter both sides of each round and toast both sides in a sauté pan over low heat until golden.
4. When bread is toasted and stew ripened, return stew to gentle heat and bring to just below a simmer. Taste and carefully add salt, which brings out the sweetness of oysters.
5. Place a piece of toast in the bottom of each bowl and ladle soup over, taking care to distribute oysters evenly between portions.