Method

• 1.
  Heat oil and butter in a large, deep frypan over medium-high heat. Once butter melts and is foaming, add the zucchini and cook for 2-3 minutes, turning occasionally, until cooked and golden. Use tongs to transfer to a plate and cover to keep warm. Add asparagus to same pan and cook, tossing occasionally, for 1 minute or until just tender, bright green and golden. Use tongs to transfer to plate with zucchini, cover to keep warm and set aside.
• 2.
  Add leek and garlic to the same pan and reduce heat to medium. Stir through spinach and cook, tossing occasionally, for 1 minute or until spinach wilts. Remove pan from heat and return zucchini and asparagus to pan.
• 3.
  At the same time, cook the spaghetti in a large saucepan of boiling, salted water according to packet directions (about 6-8 minutes) until al dente, reserving 3/4 cup (180ml) pasta water.
• 4.
  Meanwhile, whisk yolks, parmesan and 2 tbs cold water in a large jug. Season well. This is where you need everything ready to go, so that you can work quickly. Use tongs to transfer pasta from saucepan to frypan with the vegetables, and toss well to combine. Temper the egg yolk mixture with 1/2 cup (125ml) hot leftover pasta cooking water, whisking with a fork to combine, then pour over the pasta in frypan and toss quickly to coat and melt the cheese, and to work the starch in the pasta (this will help thicken the sauce).
• 5.
  If the carbonara looks a little dry, add more of the leftover pasta cooking water to loosen, up to another 1/4 cup (60ml). Toss through the lemon zest and juice and season well. Serve immediately, sprinkled with extra parmesan.