For the venison, heat a little oil and the butter in a thick-bottomed pan. Season the venison with salt and freshly ground black pepper. When the butter is foaming add the venison and sear the meat on all sides. Cook for approximately ten minutes for medium-rare then remove from the heat and leave to rest for a further 15 minutes.
For the ragout, melt 40g/1½oz butter in a pan and add the button onions. When the onions are lightly coloured, add the lardons and brown well. Pour the water into the pan, just covering the onions and lardons. Cover the pan with a piece of buttered greaseproof or parchment paper and cook until the water has reduced and the onions are soft.
Once the onions are cooked, add the peas and broad beans and cook for a further two minutes. Whisk in the remaining butter to thicken the sauce and add the chopped rosemary.
To serve, slice the venison. Place a little ragout into the middle of four serving plates and place the venison on top. Pour more of the sauce over and around.