Prepare the tomato sauce by combining the passata sugar and seasoning in a saucepan. Heat over a medium-low heat until it comes to a simmer. Cook this way until it has thickened a little. Remove and set to one side.
Heat the olive oil in batches in a frying pan and sauté the peppers with a little seasoning for 2-3 minutes, tossing frequently, until they start to soften. Move to a bowl, then add a little more olive oil to the frying pan and sauté the aubergine with a little salt for 2-3 minutes, tossing frequently. Move to a separate mixing bowl. Add a little more olive oil to the frying pan and saute the courgette with a little seasoning for 2-3 minutes, tossing frequently. Move to the same mixing bowl as the aubergine.
Add some olive oil to the pan and sauté the onion and garlic for 2-3 minutes, until just starting to soften, making sure you stir and toss frequently. Toss all the vegetables together in a large mixing bowl until evenly combined.
Bring a large saucepan of salted water to the boil and cook the lasagne sheets, in batches, until tender. Drain and dry off.
Preheat the oven to 180°C.
Lay the sheets of lasagne on a flat surface and spoon the ratatouille into the centres. Roll the lasagne up and around to create the cannelloni. Carefully arrange seam-side down in a lined roasting tray. Drizzle with any remaining olive oil and bake for 10 minutes.
Meanwhile, reheat the sauce.
Remove the cannelloni after 10 minutes and carefully lift one of each variety onto serving plates and sit them flush next to one another. Spoon any remaining vegetables from the tray onto the plates. Spoon the sauce on top and finish with a sprinkle of parsley on top of the sauce. Serve immediately.