Cook the lentils in the vegetable broth for 25-30 mins.
Preheat the oven to 400 F. In a large skillet, heat the olive oil over medium-high heat and cook the onion, carrots, celery, and mushrooms until tender. Sprinkle the flour over the veggies, and cook for one minute.
Add the lentils with the vegetable broth, tomato paste, red wine, frozen peas, chopped parsley, oregano, and mustard powder. Bring to a boil, reduce heat, and simmer for 10 minutes. Season with salt and pepper.
Place the lentil mixture in a deep pie dish and top with the mashed potatoes. Bake for 25-30 minutes or until the mashed potatoes are slightly browned.