Preheat the oven to 200 degrees F. Place the turkey, egg, sage, onion, garlic, bread crumbs, salt and pepper in a bowl and gently mix to combine. Form the mixture into four, 1 inch thick, patties. Heat the oil in a non-stick skillet set over medium heat. Cook burgers 4-5 minutes per side, or until entirely cooked through. Transfer to a heatproof plate and keep warm in the oven. Place the stock and flour in a bowl and whisk to combine. Drain the excess fat from the skillet, and then pour in the stock/flour mixture. Bring to a simmer and cook until the gravy thickens; season with salt and pepper. Place the ground turkey steaks on plates and serve the gravy alongside for spooning on top.