Wrap a slice of bacon around each beef medallion and secure with toothpicks. Season with salt and pepper. In a skillet, heat the oil until shimmering. Add the medallions and cook over moderately high heat until browned on the bottom, 2 minutes; turn and cook for 1 minute. Add the morels and scallions and cook over moderate heat until the scallions are tender. Transfer the meat to a plate. Cook the vegetables for 2 minutes more, add the Madeira and simmer for 2 minutes. Add the meat and stock and simmer over moderately low heat, turning, until an instant-read thermometer inserted in the center of the meat registers 140°, about 3 minutes. Transfer the meat to plates; discard the toothpicks. Remove the skillet from the heat and swirl in the butter. Season with salt and pepper, spoon over the meat and serve.