Preheat the oven to 220°C.
Heat vegetable oil in an ovenproof frying pan and brown the seasoned venison on all sides.
Transfer the pan to the oven and roast the venison loin for 15 minutes.
Heat butter and olive oil in a second frying pan and add shallots, squash, and rosemary.
Roast the vegetable mixture in the oven for 15 minutes until tender.
Blend par-boiled squash, mustard, and cream in a processor to create a smooth purée.
Boil the port, red wine, stock cube, and vinegar in a pan, then simmer until thickened.
Plate the mash and top with vegetables, blanched mangetout, venison, and sauce.