Treacle-Cured Beef with Yorkshire Puddings and Roast Potatoes

7 ounces black treacle (see Cook's Note) 1 3/4 pounds fillet of beef (a middle cut such as rib or loin) Salt and freshly ground black pepper 3 to 4 tablespoons vegetable oil 2 knobs of butter 8 potatoes, peeled and cut into large chunks 1 1/8 pounds spinach 2 1/4 cups red wine sauce or gravy, for serving