Heat half the olive oil in a large pan and tip in the onion and sauté for 2-3 minutes until softened but not coloured. Stir in the minced beef and cook for another few minutes until lightly browned, breaking up any lumps with a wooden spoon. Stir in the passata and add the sugar and herbs, then season to taste and simmer for 15-20 minutes until slightly reduced and thickened.
Meanwhile, wash the spinach and remove any large stalks, then dry well – a salad spinner works brilliantly if you have one. Heat a large heavy-based pan and add fistfuls of the spinach, adding another as one wilts down. Cook for 1 minute, then tip into a colander and gently press out all the excess moisture.
Heat the rest of the olive oil in the pan and sauté the garlic for 30 seconds then add the drained spinach and season to taste. Toss until heated through and then tip into a colander to drain.
When the Bolognese sauce is almost ready, cook the gnocchi in a large pan of salted water for 5 minutes or according to packet instructions. Mix the mascarpone with the Parmesan and season with pepper. Squeeze out any remaining excess moisture from the spinach. Stir the gnocchi and spinach into the Bolognese sauce and tip into a large ovenproof dish. Add dollops of the mascarpone mixture and grill until bubbling and golden. Serve at once straight to the table with a separate basket of ciabatta bread.