Preheat oven to 350 degrees F. Season beef with salt and pepper, and then coat in flour, shaking off excess. Reserve excess flour. Heat oil in a skillet set over medium-high heat. Add steaks and brown on both sides. Transfer to casserole. Lower heat to medium. Add garlic, onions and mushrooms to the skillet and cook until tender. Stir 3 Tbsp. of the reserved flour into the pan. Stir in mustard and thyme. Slowly pour in, stirring constantly, beef stock. Bring to a simmer. Season to taste and then pour over steaks. Cover and cook in the oven 90 minutes, or until tender.