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When you're cooking any kind of steak, it's important to not throw them into the pan straight from the fridge. Let them come to room temperature first. FILL a medium-sized pan with water, add some salt and bring to a boil.
PEEL the butternut squash, remove the seeds and cut half of the squash into cubes. Set the cubes aside for later use. The other half will be used for a puree. CUT them into pieces and boil them in the second medium-sized pan in salted water for about 10 minutes.
Meanwhile, rinse the Roseval potatoes. CUT them into thick slices and cook them in boiling water for about 5 minutes. REMOVE from the water, spread them on a towel or plate to let them cool a bit.
DRAIN the squash pieces and use a fine sieve and a spoon to create a puree. Season with pepper and salt. TEAR the oyster mushrooms into pieces, finely cut the rosemary and set aside.
HEAT a combination of a little bit of butter and olive oil in a medium-sized skillet and cook the steaks for about 3 minutes on each side. Turn down the heat and cook them a bit longer, depending on how you like your steak.
REMOVE from the pan, keep warm and if needed, add a bit of extra butter to the pan. Add the balsamic vinegar and after 1 minute, also add the game stock. Let it come to a boil and reduce until you've reached the desired thickness of the sauce.
Meanwhile heat butter in another skillet and at the same time HEAT olive oil in yet another skillet. COOK the Roseval potatoes slices until they are golden brown, this will only take about 4 minutes.
SAUTE the oyster mushrooms for about 3 minutes in the skillet with olive oil. Meanwhile, reheat the squash puree and steaks if needed. DIVIDE the puree over the plates along with the oyster mushrooms and potatoes.
In the same skillet, you cooked the mushrooms, you can now again heat some olive oil and cook the squash cubes shortly. Season with rosemary, pepper and salt.
Season the steaks with pepper and salt and slice the steaks into equal pieces. DIVIDE over the plates along with the squash pieces and finish it off with the sauce.