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Heat oven to 200C/180C fan/gas 6. Heat the oil in a large frying pan and cook the onion, garlic and 2 tsp of the herbs for 3 mins until starting to soften. Add the turkey and brown all over, then tip in the tomatoes and cook for 5 mins more.
Scoop out the middle of the marrow and discard (or fry, then freeze for another time – try it mashed with potato). Arrange the slices in a baking dish. Spoon the mince into the middle of each marrow slice, then spoon the rest over the top. Cover with foil and bake for 30 mins.
Meanwhile, mix remaining herbs with the breadcrumbs and Parmesan. Remove the marrow from the oven, uncover, and sprinkle over the crumbs. Return to the oven for 10 mins more until crumbs are golden and crisp and marrow is tender.