Peel and finely chop the shallot. Pop it in a large bowl. Finely chop most of the mint leaves, keeping a few back for garnish. Add them to the shallot. Add the lamb mince to the bowl with a pinch of salt and pepper.
Use your hands to mix everything together, kneading and squeezing the mixture to combine it all. Roll the mix into meatballs around the size of a walnut. You should get around 10-12 meatballs.
Pop the meatballs on a plate. Cover with cling film or baking paper and chill in the fridge while you make the sauce.
Peel and finely chop the onion. Peel and grate or crush the garlic clove. Warm a pan over a medium heat for 1 min. Add ½ tbsp oil and the onion. Season with salt and pepper. Cook and stir over a medium-low heat for 8-10 mins till the onion is soft and golden.
Stir the garlic into onion. Cook and stir for 1 min, then pour in the passata. Cover and bring to the boil, then turn the heat down and simmer for 10 mins to mingle the flavours.
While the sauce simmers, warm a frying pan over a mediumhigh heat for 1 min. Add ½ tbsp oil. Fry the meatballs for 8-10 mins, turning often, till the meatballs are browned all over.
Pour the sauce into the meatballs pan. Simmer for 10 mins to finish cooking the meatballs and thicken the sauce.
While the meatballs cook, separate the lettuce leaves and shred them. Whisk 1 tbsp balsamic with ½ tbsp olive oil and a pinch of salt and pepper. Toss with the lettuce leaves.
Heat your grill to high. Grill 2 piadinas for 1-2 mins on each side till a little browned and soft. Pop them on 2 plates. Spoon the meatballs and their sauce over the top of the piadinas. Garnish with the remaining mint and serve with the lettuce.