2. Arrange the meat in a broiler pan and place under broiler, such that it stays 2 inches away from heat and broil it for 5 minutes on each side.
3. In a saucepan, melt butter and cook onions until they turn translucent and soft.
4. Add condensed golden mushroom soup, water, chopped canned tomatoes, chopped parsley and Dijon mustard to the saucepan and cook this sauce until it thicken, by stirring it at occasional intervals.
5. Allow the meat to cool, and slice them thinly and diagonally before arranging them across the grain.
6. Serve London Broil with Mushroom Tomato sauce.