Preheat oven to 180C. Add the marrow bones to a baking tray, sprinkle with salt and thyme and roast for 30mins. Remove from oven and scoop the soft marrow & leaves from thyme sprigs into a food processor with the butter. Blend until smooth, taste and season if required. and store in jars in the fridge. Can also be frozen for 3-6 months. Use discarded bones for broth.