Preheat the oven to 180C/fan 160C/Gas 4. Line the base and sides of a 1.5 litre loaf tin with parchment paper.
Heat the oil and butter in a large frying pan and cook the onion and celery for about 5 mins until beginning to soften. Stir in the garlic and mushrooms and cook for a further 10 mins.
Stir in the red pepper and grated carrot and cook for about 3 mins then add the oregano and paprika and cook for just a minute.
Add the red lentils and tomato puree and cook for about 1 min, then add the vegetable stock and simmer over a very gentle heat until all the liquid has been absorbed and the mixture is fairly dry. Set aside to cool.
Finally, stir in the breadcrumbs, nuts, eggs, cheese and parsley and a pinch of salt and some ground black pepper. Stir to mix well then spoon the mixture into the prepared tin and press down the surface . Cover with foil and bake for 20 mins, then remove the foil and bake for a further 10–15 mins until firm when pressed gently.
Meanwhile, to make the sauce, heat the oil very gently then add the garlic slices and rosemary sprig and heat without colouring. Pour in the passata and add a pinch of salt and some ground black pepper. Simmer gently for just 15 mins.
Allow the loaf to cool in the tin for about 10 mins then turn out onto a serving board or plate. Remove the baking paper and cut into slices and serve with a little of the tomato sauce.