Instructions:
- Preheat oven to 425°F (220°C). Line a sturdy rimmed baking sheet with parchment paper or foil.
- Place the bone marrow pieces cut-side up on the prepared sheet. Sprinkle generously and evenly with sea salt and black pepper. If using, nestle a thyme sprig beside each bone.
- Roast for 20–25 minutes. The marrow should look slightly bubbling around the edges and be pale golden brown on top. A knife should meet almost no resistance.
- While the marrow roasts, brush both sides of the bread slices with olive oil. Toast the bread until golden and crisp, either in a dry pan or in the oven for the last 5 minutes of the marrow cooking time.
- In a small bowl, combine the finely chopped parsley, lemon zest, chopped capers, and minced garlic to create the Gremolata mixture.
- Carefully remove the baking sheet from the oven. Arrange the hot marrow bones and toasted bread on a serving platter.
- Allow guests to spoon the hot, glistening marrow onto their toast, then top immediately with a sprinkle of the bright Gremolata mixture. Serve immediately!