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Mix the beef mince with the breadcrumbs, lemon zest, egg, half of the garlic, the fennel seeds, herbs, plenty of freshly ground black pepper and ½ tsp salt in a bowl. Knead well to combine, then roll into 12 evenly-sized balls (weighing around 60g each). Press your thumb deep into each ball to make a pocket, then fill each one with a piece of mozzarella and seal inside by rolling the meatball into shape again.
Fry the meatballs over a medium-high heat in half the olive oil for 5-10 mins, turning regularly until well-browned with a good crust. Meanwhile, heat the remaining olive oil in a saucepan over a medium heat and cook the remaining garlic for 2 mins until fragrant. Tip in the passata or tomatoes, season well and cook for 10 mins until thickened.
Add the meatballs to the sauce and gently simmer for 3-5mins, basting the meatballs in the sauce. Remove from the heat, then leave to rest for 5 mins.
To serve, cut along the length of the baguette but not all the way through, then open it up and scoop in the meatballs and sauce. Sprinkle over the parmesan, press the baguette shut and cut into four pieces to serve.