Bring a medium saucepan of salted water to the boil for the pasta. Coarsely chop the walnuts. Crush or finely chop the garlic. Finely chop half the chilli*, discarding the seeds if less heat is desired. Finely grate the zest of half the lemon*, then juice the half. Finely grate the parmesan.
Step 2 — Cook pasta and peas
Cook the pasta in the pan of boiling water for 9 mins. Add the peas and cook for a further 2 mins or until the pasta is al dente. Drain.
Step 3 — Toast walnuts
Meanwhile, put the walnuts in a cold large deep frypan over medium heat. Toast, tossing, for 3-4 mins until evenly golden. Remove from the pan. Crumble 1 stock cube* into a heatproof jug, add 80ml (⅓ cup) boiling water and stir to dissolve.
Step 4 — Start sauce
Heat 1 tbs olive oil in the same pan over medium heat. Cook the garlic and chilli (if preferred, omit the chilli when cooking and serve it at the table), stirring, for 30 secs or until fragrant.
Step 5 — Finish sauce
Add the stock and half the cream* to the garlic mixture and cook, stirring, for 1 min or until heated through. Add the pasta, peas and three-quarters of the parmesan to the sauce and stir to combine.
Step 6 — Get ready to serve
Add the spinach, lemon zest and 1 tbs lemon juice to the pasta, stir to combine, then remove the pan from the heat. Taste, then season with salt and pepper. Divide the pasta among bowls. Scatter over the walnuts and remaining parmesan to serve.