Peel the onion and carrot. Rinse the celery. Finely dice them. Heat 2 tbsp olive oil in a large pan. Add the veg. Pop on the lid. Fry gently over a low heat for 10-15 mins till soft, stirring every now and then.
While the veg cook, peel the garlic. Roughly chop it. Rinse a sprig of rosemary. Remove the leaves from the stalk. Finely chop. Rinse the leaves from 1/2 the lettuce. Pop in a salad spinner or pat dry with kitchen paper. Pop in a bowl. Fill the kettle and boil it.
Take the lid off your pan. Turn up the heat to high. Add the lamb mince, garlic and rosemary. Season with a little salt and pepper. Fry for 5 mins till the lamb is browned. Stir every now and then.
Stir in the tomatoes and turn the heat down to medium. Fill the tomato tin a quarter full with hot water from your kettle. Add to the sauce. Simmer for 10 mins till the sauce thickens. Stir every now and then. Grate half the Cheddar.
Heat your oven to 200ºC/Fan 180ºC/Gas 6. Fill a pan with hot water from the kettle. Add a pinch of salt. Cover. Bring to the boil. When it’s boiling, add the penne. Simmer for 5 mins. Drain.
Taste the sauce. Adjust the seasoning. Stir in the drained pasta. Tip into a baking dish. Top the pasta with the grated Cheddar. Bake for 20-25 mins till the cheese is golden and bubbling.
Dress the lettuce with 1 tbsp olive oil and a pinch of salt and pepper. Serve the pasta bake on warmed plates with the dressed lettuce.