Season the loin with salt and pepper and pan-roast in a little oil over a moderately high heat for 3–4 minutes. Add a knob of butter, and continue to cook for another couple of minutes while continually basting with the butter until caramelized. Add sprigs of thyme and rosemary. Cook until medium-rare, but more on the rare side (internal temperature of 128-130 degrees). If over cooked, the venison could taste “gamey.”
Remove the venison from the heat and rest in a warm place for 10 minutes.
While the meat is cooking, slice the potato into 4 rounds ¼ inch thick and place in sauté pan with chicken stock and cook on medium heat until potatoes are tender, about 15 minutes. Discard the remaining stock and add two tablespoons of butter to the pan , cooking on medium high heat until potatoes are crisp, flipping once to crisp both sides.
Carve the loin into 4 even pieces and season the cut surfaces. Serve the deer on top of the potatoes.