Add lemon juice to taste, plus sugar, a pinch of salt, and one grind of black pepper. Refrigerate. Shortbread: Place the butter and confectioner’s sugar in a standing mixer and beat until creamy.

Season well with salt and freshly ground black pepper and place into the fridge to marinate overnight. For the sauce, heat a non-reactive frying pan until hot. Add the butter and onions and fry ...