Cut the chicken thighs into 1 and 1/2 inch cubes. Season with salt and pepper. Mix well. Add the starch and toss to each piece thoroughly.
Heat the cooking oil in a wok or frying pan. While the oil heats, start making the sauce.
Put the lemon juice and sugar in a small pan. Bring to the boil, swirling the pan as the sugar starts to melt. Lower the heat to medium and allow the sauce to boil until reduced and thick. This takes anywhere from five to 10 minutes depending on the amount of juice (some lemons are larger and juicier than others) and the moisture in the sugar.
When the oil starts is hot enough, fry the chicken cubes (in batches if your pan is not large enough) until golden and crisp outside. Scoop out and drain on paper towels.
In a clean pan, toast the walnuts. Set aside.
Pour off the oil (and scrape and residue) from the pan where you cooked the chicken. Reheat the pan. Add the carrot pieces. Stir around for about 30 seconds. Pour in the sauce. Add the cooked chicken. Throw in the walnuts too. Toss everything to distribute the sauce well. You want every piece of chicken and walnut to be coated with the thick glaze.
Tip the contents of the pan onto a serving platter. Sprinkle the scallions on top and serve at once.