1. Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius).
2. Heat oil in a large skillet. Add onion and sauté for about seven minutes, or until starting to brown. Finely dice the mushroom stems and add to the onion, stirring well. Cook until browned and any liquid has evaporated, about 10 minutes.
3. Add tomato, tomato paste, salt, pepper, mustard, and parsley. Cook another two minutes. Remove from heat and allow to cool for 10 minutes.
4. Gently mix in lamb. Stuff mushrooms with the mixture.
5. Transfer to a lined baking pan and bake for 30 minutes.