Preheat the oven to 350 degrees F. Combine first 5 ingredients and 1 tsp of the thyme in a bowl. Form into 1-inch balls and place on non-stick baking tray. Bake 20-25 minutes. Meanwhile, heat the oil in an oven-proof pot over medium heat. Add the vegetables and cook 4-5 minutes. Mix in the flour until well combined. Slowly mix in the beef stock, stirring constantly. Add remaining thyme, bay leaf and mustard. Bring to a simmer. When the meatballs are cooked drain and add to the pot. Cover and cook stew in the oven 1 hour. Stir in peas, season with salt and pepper, and serve.