1. Crumble beef into casserole dish. Microwave, uncovered, on High for 5 to 7 minutes, until beef is no longer pink, stirring twice and using a fork to break up meat lumps. Drain off any excess fat in dish.
2. Add onion, mushrooms, garlic and thyme to beef in casserole dish. Microwave, covered, on High for 3 to 5 minutes or until vegetables are softened.
3. Place flour in a bowl and blend in 1/3 cup (75 mL) of the stock until smooth. Stir into beef along with remaining stock, tomato paste, Worcestershire sauce, salt, and pepper to taste. Microwave, covered, on High for 4 to 6 minutes or until sauce boils and thickens. Stir in peas.
4. Crust: In a bowl, combine biscuit mix and half of the cheese. Using a fork, stir in enough milk to make a soft, sticky dough. Turn out onto a lightly floured board and shape into a ball. Pat dough into a circle or square large enough to fit casserole dish.
5. Place dough on top of warm meat mixture. Sprinkle top with remaining cheese. Bake in preheated oven for 20 to 25 minutes or until top is golden.