Preheat oven to 200C/400F fan/gas mark 6.
For the filling, put the spinach in a large colander and pour over a kettle of boiling water to wilt it (you may need to do this in batches). When cool enough to handle, squeeze out the excess water, roughly chop, and mix in a large bowl with the mascarpone. Season well with salt and pepper.
For the béchamel sauce, melt the butter in a pan over a medium heat and stir in the fl our. Remove the pan from the heat and whisk in the milk, a little at a time. Put back on a low heat and continue whisking until the sauce thickens. Stir in the nutmeg and set aside.
Finely chop the parsley, grate the Parmesan, and mix with the breadcrumbs.
Divide the passata between the two dishes and sprinkle over the oregano.
Using a piping bag or plastic food bag with the corner snipped off, squeeze the mascarpone filling into the cannelloni tubes. Lay the tubes, side by side, on top of the tomato sauce and spoon over the béchamel sauce. Top with mozzarella and breadcrumbs, and bake for 30-35 mins until golden and bubbling. Remove from the oven and let stand for 5 mins.
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