Spray a large frying pan with low-calorie cooking spray and place over a medium heat. Add the onion and cook gently for 10 minutes until softened. Add the skinned sausages to the frying pan and break them up into small pieces using a wooden spoon. Stir to combine with the onion and cook for about 5 minutes, or until the sausage meat is cooked through and the mixture is lightly browned. Place in a medium bowl and set aside.
Place the spinach in a large saucepan, cover with a lid and place over a low heat and cook gently for about 2-3 minutes, or until the spinach has just wilted. Place a sieve over a bowl, add the wilted spinach and squeeze out the excess liquid with the back of a spoon. Discard the liquid. Leave the spinach to cool a little.
Place the spinach in the bowl with the onion and sausage meat. Add the ricotta cheese, Parmesan, garlic granules and season with salt and black pepper, to taste. Stir until completely combined and set aside.
Place the passata in a medium bowl with the garlic granules, dried Italian herbs and season to taste with salt and black pepper. Stir until combined and set aside. Preheat the oven to 180°C.
Pour one third of the tomato sauce into an 18 x 27cm ovenproof dish and spread out evenly. Add half of the filling mixture and spread out roughly with a fork.
Place half of the lasagne sheets on top of the filling mixture, trimming to fit if needed. Pour half of the remaining tomato sauce over the lasagne sheets and spread out evenly.
Add the remaining filling mixture and spread out roughly with a fork. Place the remaining lasagne sheets on top of the filling mixture.
Pour over the remaining tomato sauce and spread out to completely cover the lasagne sheets. Sprinkle the top with finely grated Cheddar cheese and place on a baking tray. Bake in the preheated oven for 30-35 minutes, until the lasagne is soft when tested with a sharp knife and the top is golden.