Preheat oven to 190°C. Put 30g of the butter, garlic and leek in a medium saucepan over a medium heat. Cook for 15 minutes or until softened and coloured slightly. Set aside.
Put stock and bay leaf in a medium saucepan over a medium heat and bring to a simmer. Meanwhile, put remaining butter in a separate medium heavy-based saucepan over a medium heat. Cook until butter is light brown and bubbles subside. Add flour and whisk to combine. Gradually whisk in hot stock, in a steady stream. Cook for 3 minutes then remove from heat and add cream and lemon juice. Season and set aside to cool for 10 minutes.
Stir leek mixture through white sauce and add capers.
To assemble, spread a little of the sauce in a 20 x 30cm baking dish. Season and scatter over ¹⁄³ of the crabmeat. Dollop with a ¹⁄³ of the mascarpone, bocconcini and parmesan. Top with 4 pasta sheets, then repeat layering until all ingredients are used, finishing with a layer of sauce and a sprinkling of parmesan.
Cover with baking paper and a layer of foil. Bake for 40 minutes. Remove paper and foil and cook, uncovered, for a further 10 minutes or until browned and crisp. Serve.