Place the bone marrow shafts, bone side down into a large baking dish. Sprinkle the crushed garlic and some seasoning along the shaft then place in the oven and roast for about 15 minutes. The bone should just be turning gold at the edges and the marrow glistening. Remove from the oven
Whisk together the Dijon mustard, lemon juice and seasoning and then pour in the olive oil in a slow steady stream, whisking it in until fully emulsified.
Toss the parsley and sorrel together then drizzle the dressing over and serve with the hot bone marrow.