Bring a medium saucepan of salted water to the boil for the pasta. Finely chop the onion. Finely grate the parmesan. Finely grate the zest of half the lemon*, then juice the half. Coarsely grate the haloumi and zucchini, keeping them separate.
Step 2 — Start sauce
Heat 2 tsp olive oil in a medium deep frypan over medium heat. Cook the onion, stirring, for 5 mins or until soft. Increase the heat to high, add the lamb mince and cook, breaking up the lumps with a spoon, for 3-4 mins until browned. Stir in the zucchini.
Step 3 — Cook pasta and sauce
Cook the pasta in the pan of boiling water for 10 mins or until al dente. Drain. Reserve the pan. Meanwhile, add the tomatoes, tomato paste, Greek seasoning and 80ml (⅓ cup) water to the mince mixture and bring to a simmer. Reduce the heat to medium-low and cook, covered, for 8-10 mins until reduced. Stir in 1 tsp sugar. Taste, then season with salt and pepper.
Step 4 — Make bechamel
While the pasta and sauce are cooking, preheat the grill to high. Melt 20g butter in a small saucepan over medium heat. Add 2 tsp flour and cook, stirring, for 2 mins or until foaming. Gradually add 60ml (¼ cup) milk, stirring constantly to prevent lumps forming. Bring to a simmer and cook, stirring, for 3 mins or until thickened. Stir in the parmesan.
Step 5 — Grill pastitsio
Bring some water to the boil in the reserved pan for the beans. Lightly grease a medium ovenproof dish. Put the pasta in the dish. Spoon over the sauce, then spread with the bechamel and scatter with the haloumi. Grill for 10 mins or until the haloumi is melted and golden brown.
Step 6 — Cook beans and serve
Meanwhile, trim the beans. Cook the beans in the pan of boiling water for 3-4 mins until tender. Drain. Combine the lemon zest, 2 tsp lemon juice* and 1 tsp extra virgin olive oil in a bowl and season with pepper. Add the beans and toss to coat. Serve the pastitsio at the table with the beans.