The mash can be completely made and the shallots cooked up to 2 days ahead ready to be reheated in a pan or microwave mixed together.
Heat a large frying pan. Melt a knob of the butter and add the shallots and thyme sprigs. Cook over a low heat for 20 mins, stirring occasionally, until soft and golden. Add the sugar and cook 5-10 mins more until light brown.
Meanwhile, boil the potatoes for 15 mins or until soft. Gently heat the milk and remaining butter in a small pan, just until boiling, then drain the potatoes well and return to the cooking pan.
Mash until smooth, then stir in the warm buttery milk and season well. Tip most of the shallots into the mash, discarding the thyme stalks and mix in. Top with the leftover shallots and serve straight away.