Cook the penne in a large saucepan of boiling water for 10–12 minutes, or according to the packet instructions, until al dente.
Meanwhile, heat the oil in a large frying pan over a medium-low heat, add the spring onions and fry for 1–2 minutes or until softened. Stir in the mascarpone, lemon juice, peas, broad beans and basil. Season with salt and pepper to taste and stir for 1–2 minutes or until bubbling.
Drain the penne, reserving 3 tablespoons of the cooking water. Stir the pasta and the reserved liquid into the creamy peas and beans. Serve immediately, garnished with extra basil leaves.