Heat the oil and butter together in a large, deep frying pan. Add the onions and fry over a medium heat for 5 minutes, until softened.
Add the garlic and rice, and stir until they are coated in the oil and butter.
Stir in the stock and half of the mixed herbs, cover and simmer for 15 minutes, stirring occasionally, until most of the liquid has absorbed.
Meanwhile, empty the tin of tomatoes into a medium bowl, add the remaining mixed herbs and whizz together with a handheld blender.
Stir the blended tomatoes into the stock, cover and simmer for a further 5 minutes, until the rice is tender.
Meanwhile, cook the broccoli florets in a medium pan of boiling water, for 3-4 minutes, then drain.
Remove the risotto from the heat. Stir through the soft cheese, then the broccoli and mozzarella. Season with freshly ground black pepper, to taste, and serve immediately.