Béchamel: melt butter (4 tbsp) over medium heat, take off heat, whisk in flour (30 g) 30 sec. Back on low heat, whisk in milk (875 ml) 1 cup at a time until smooth (thin with more milk if needed). Raise to medium, cook 6-7 min stirring until thickened and coating the spoon. Off heat, stir in parmesan (50 g); season with salt and pepper.
Preheat oven to 180°C. Spread ~1 cup meat sauce in a 9x13-inch dish, cover with lasagne sheets, add ~1 cup meat sauce and 1 cup bechamel (50 g). Repeat layers. Top final sheets with remaining meat and bechamel sauces and chunks of mozzarella.
Bake 25-30 min until golden and bubbling. Garnish with parsley, rest 10 min, then slice and serve.