Thinly slice the zucchini (courgettes) lengthways using a mandolin or knife. You want the slices relatively thin (approximately 5mm), but thick enough that they won't disintegrate when cooked.
Heat a non stick frying pan over a medium low heat. Add some olive oil to a piece of paper towel and rub over the frying pan to lightly grease. Cook the zucchini on each side until cooked and lightly browned. Place on a plate to cool while you cook the remaining slices.
Once the zucchini is cooked add olive oil to the pan and add the onion and garlic, and cook until the onion is soft. Add the silver beet stalks and cook until softened before adding the leaves. Cook until the leaves are wilted. Add the lemon juice. Leave to cool.
Preheat the oven 180 celsius (350 Fahrenheit). Place the ricotta in a large bowl along with the ½ cup of cheese, herbs, nutmeg, silver beet, onions and garlic. Mix to combine and season to taste.
Take 3 slices of zucchini and lay on a chopping board lengthways so that they are slightly overlapping. Take a spoonful of the ricotta mixture and place along the short edge on the zucchini and roll up the zucchinni to cover the filling. Place in a lightly greased large (20cm x 28cm) baking dish. Repeat with the remaining mixture.
Top with the tomato passata or pasta sauce, if using, and top with remaining cheese. Bake for 20 to 25 minutes, or until the cheese is golden.
The cannelloni can be prepared up to 1 day ahead. Top with the passatta and cheese just before baking.