For the tomato sauce, heat the oil in a saucepan and gently fry the onion and garlic until the onion is translucent, about 4-5 minutes.
Add the white wine and simmer for about one minute, until slightly reduced. Add the tomatoes and simmer for a further five minutes. Season, to taste, with salt and freshly ground black pepper and remove from the heat. Process until smooth with a hand blender.
For the potato and sausage, heat the oil in a separate frying pan and fry the cubes of potato and the sausage halves until coloured and cooked through, about 7-8 minutes.
To serve, pile the potatoes and sausages onto a serving plate and pour over the tomato sauce. Garnish with chervil.