Step 1 Put stock cubes in a heatproof jug with 4 cups boiling water.
Step 2 Heat half the olive oil in a large saucepan set over medium heat. Add mushrooms in batches and stir for 2 minutes, or until golden; transfer to a plate and set aside. Add remaining olive oil with leek; cook, stirring, for 5 minutes, or until soft. Add rice; cook, stirring, for 1 minute, or until well coated in oil and slightly toasted.
Step 3 Add 1/3 cup hot stock to rice mixture and stir until rice absorbs stock; continue for 15 minutes, or until rice is tender and creamy, reserving 1/3 cup stock.
Step 4 Add reserved mushrooms to rice mixture with peas, spinach and reserved stock; cook, stirring, for 3 minutes, or until peas are just tender and spinach wilts.
Step 5 Scatter mushroom risotto with chopped walnuts, thyme leaves and lemon zest, and serve.