Preheat the oven to 200ºC, fan 180ºC, gas 6. In a small saucepan, bring the ketchup, sugar, soy sauce and ginger slowly to the boil over a low heat, stirring to dissolve the sugar. Reduce the heat and simmer for 5 minutes, stirring often until thickened. Put the chicken thighs, potatoes and shallots in a large roasting tin.
Pour over the ketchup glaze and toss well so that everything is well coated.
Bake in the oven for 1 hour, covering the tray with foil after 40 minutes, until cooked through with no pink remaining. Serve with the salad leaves.