- Melt butter in a heavy based saucepan, add onion and garlic and cook until softened. Add tomato sauce and cook for 10 minutes, stirring occasionally, until slightly thickened. Season to taste.
- Grate half the Parmesan and combine with cooled mashed potato, ricotta, eggs, herbs and seasonings. Knead in enough flour to obtain a smooth, non sticky dough (amount will vary, depending on the dryness of potatoes).
- Divide dough into 4 equal portions and roll each into a long sausage shape 2cm thick. Cut into 2cm lengths. If desired, gently roll each gnocchi over the back of a fork.
- Bring a large saucepan of salted water to the boil and cook gnocchi in batches until they rise to the surface. Toss immediately with hot sauce, asparagus and basil leaves. Serve with remaining shaved or grated Parmesan and a drizzle of olive oil.