For the venison, the day before cooking, place the venison in a dish and cover with three tablespoons of the olive oil, and all of the balsamic vinegar, carrot and onion. 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 the onion for 2-3 minutes, until softened.
Add the morels and cook for 10 minutes.
Add the porcini and fry for a further 3-5 minutes.
Add the balsamic vinegar, sherry and cream and simmer for 10 minutes. Season, to taste, with salt and freshly ground black pepper. (The sauce may become quite thick, so add a little of the morel soaking water to the pan.)
To cook the venison, remove the venison from the marinade, pat dry with kitchen paper, then season well with salt and freshly ground black pepper.
Heat a separate frying pan until hot. Add the remaining two tablespoons of olive oil and the venison steaks. Fry the venison on each side for 3-4 minutes, until golden-brown on the outside, but still pink on the inside, or until cooked to your liking.
To serve, spoon the mushroom sauce onto warmed plates, then top with the venison steaks. Shave a little summer truffle over the top, if using.