Preheat the oven to 160°C.
Chop the washed leeks into chunky batons and slice them lengthwise. Arrange them snugly in a roasting tray lined with baking paper. Remove the inner core of each leek to create a small pocket.
Mix the Barbaros Nduja with the softened butter (add lemon zest if desired) until well combined. Transfer to a piping bag or ziplock bag.
Pipe the nduja butter into the cavity of each leek. Drizzle with olive oil and season with flaky sea salt.
Wrap the tray tightly with foil and bake for 20 minutes.
While the leeks are baking, finely slice the removed leek cores and cook them in a small pan with 1 tbsp Barbaros Beef Tallow until sizzling and crispy. Drain on paper towel and set aside.
In the same pan, heat 1 tbsp Barbaros Beef Tallow, then add the breadcrumbs, chopped parsley and lemon zest. Cook, stirring, until golden and crunchy. Season with flaky sea salt.
Remove the leeks from the oven and transfer to a serving plate.
Top with the crispy leeks and pangrattato, then serve.