1. In a 2 1/2- to 3-quart pan over medium heat, stir milk often just until steaming, about 6 minutes; do not boil.
2. Meanwhile, drain oysters (reserve liquor) and cut into bite-size pieces. Add oysters and liquor to milk and occasionally stir gently just until heated through, 3 to 4 minutes. Add salt and pepper to taste.
3. Ladle stew evenly into four wide, shallow bowls and top each with 1/2 tablespoon butter.