Preheat oven to 500 degrees.
Heat a large oven-proof skillet over high heat. Add the clams, onions, curry powder, and butter. Toss until the onions start to brown.
Top the clams with all the remaining ingredients except the heavy cream, scallions and crostini. Bring to a boil, transfer to the oven and roast for 9 minutes, turn the clams and roast for about 9 to 10 minutes longer, until they pop open. Layer crostini on platter.Transfer the clams and most of the solids on bread. On the stovetop, over a medium high flame, add the cream to the liquid left in the skillet. Stir constantly until the cream is completely incorporated. Pour liquid over clams, top with scallions and serve immediately.