1) Freeze the chicken breasts for 20 to 30 mins and then thinly slice the chicken against the grain into strips. Toss the strips with about half the garlic and ginger, the soy sauce, sugar, 3g of the cornflour, 6g of the salt, and the sherry in a bowl. Marinate at room temperature for 15 mins. Mix the remaining cornflour with the stock.
2) Heat 1 tbsp of the oil a large nonstick skillet over high heat. Add the asparagus, spring onions, remaining garlic, ginger, 60ml of water, and season with a pinch of salt. Stir-fry until the asparagus is bright green but still crisp, about 3 mins. Transfer to a bowl.
3) Heat the same skillet until very hot, and then add remaining 1 tbsp oil. Add the chicken and stir-fry until it loses its raw colour and gets a little brown, about 3 mins. Return the asparagus to the pan and toss to heat through. Stir in the reserved cornflour mixture and bring to a full boil to thicken. Mound the stir-fry on a serving platter or divide among 4 plates; serve with rice.