Bring a very large pot of salted water to the boil.
Pat chicken dry with paper towels and season with salt. Heat a drizzle of oil in a large fry pan on high heat. Cook chicken, in batches, for 3-4 minutes each side, until browned (they do not have to be cooked through). Remove from pan and set aside on a plate, covered.
While chicken is cooking, cook pasta as per packet instructions. In a small bowl, combine hot stock and cornflour until dissolved.
Heat a drizzle of oil in same pan used for chicken on medium-high heat. Cook leek and silverbeet stalks for 3-4 minutes, until softened. Add 1-2 tablespoons water in the last 30 seconds to help vegetable steam. Roughly chop chicken.
Stir through stock/cornflour mixture, silverbeet leaves, peas, sour cream, milk, basil pesto, salt and chicken, along with any resting juices. Bring to a simmer and cook on medium for 2-3 minutes until chicken is just cooked through.
Drain pasta well and return to pot, along with chicken and sauce. Toss to combine and season to taste with salt and pepper.
To serve, divide chicken pesto pasta between bowls.
*To make basil pesto place 1½ cups packed basil leaves, ½ cup flat-leaf parsley (leaves and stalks), ½ clove garlic finely chopped, ¼ cup toasted pine nuts (optional), ¼ cup grated parmesan in a food processor and blitz to combine, while slowly drizzling in 1/3 cup extra virgin olive oil. Season to taste with 1-2 tablespoons lemon juice, salt and freshly ground black pepper. The pesto will keep in the fridge for a few days or can be frozen in a sealed ziplock bag for a few months. Makes 1 cup.