Carefully pour out most hot oil from skillet in which you cooked beef and oysters, leaving as many crumbly bits as possible on bottom of pan. Add flour to skillet and cook over medium-high heat, stirring with crumbly bits and oil remaining in pan, 3 to 4 minutes. Add onion, green onions, salt, pepper and Essence and saute until vegetables are tender, 4 to 5 minutes. Whisk in milk and stock little by little, until thoroughly combined. Cook until sauce is smooth and thickened, 5 to 10 minutes. Serve hot with Chicken Fried Steak and Oysters.