Preheat oven to 350 degrees. Slice the bread into thin rounds. Arrange them on a baking sheet and generously drizzle them with extra virgin olive oil. Place them in the oven and toast until golden and crisp.
Remove the crostini and raise the oven temperature to 450 degrees F. Make sure it is fully heated before adding the bones.
Rinse the marrow bones and pat dry with paper towels. Lay them flat on a baking sheet or roasting pan. Drizzle with olive oil. Place them in the oven and roast for 15 minutes.
Remove the bone and allow them to cool somewhat. Remove the marrow, add salt, pepper, and parsley and mash with a fork. Spread the mixture on the crostini.
Some folks like a squirt or so of lemon juice to balance the unctuousness of the marrow. For a more rustic presentation, present each guest with their own bones, allowing them to remove and season the marrow as they prefer.