On a large cutting board, place the Chateaubriand in front of you so that the longs sides are to the left and right and the small end is facing you. With a sharp knife, make a cut down the right side, ¾ inches in from the edge and stop ¾ inch from the bottom, be careful not to cut through the roast. Turn the roast a quarter turn and make another cut. Repeat this process until you have “unrolled” the roast like you would a sleeping bag, it should lay in one flat, even rectangle. Next sauté the shallot in the butter and add the garlic and cook for one minute add the spinach and cook until wilted. Set aside to drain. While the spinach is draining, season the inside of the roast with salt and pepper and spread the cheese evenly across the inside of the roast. Once the spinach has drained, spread it evenly over the cheese and proceed to roll the roast back into its original shape, like a sleeping bag. Tie the roast in three places with kitchen twine. In a heavy bottomed pan, sear the roast on all sides and place in a hot (400 degree) oven for twenty minutes or until an internal thermometer reads 155 degrees. Remove the roast from the oven and let rest ten to fifteen minutes and remove the twine. Slice and serve.