Preheat the oven to 200°C, gas mark 6. Put the garlic on a sheet of foil, drizzle with 1 tbsp oil and wrap tightly. Roast for 25 minutes, until really soft. Leave to cool, then squeeze the flesh from the skins. Roughly chop and set aside.
Set a large saucepan with the remaining oil over a medium heat. Cook the potatoes for 5 minutes, stirring occasionally. Stir in the chopped sage, tomato sauce and roasted garlic.
Add the lentils and stock. Bring to the boil, reduce to a simmer, then cover with a lid. Cook for 30 minutes, stirring occasionally, until the potatoes and lentils are tender.
Transfer to a blender and blitz until smooth, adding a splash of hot water to loosen if needed. Season.
Pour the soup into two warm bowls and top with mozzarella and a drizzle of olive oil. Garnish with the sage leaves and serve bread on the side, if you like.