- Rinse and drain lentils. Bring broth to a boil and cook lentils partially covered for 30 minutes over medium-low heat or until tender. Season with salt and pepper. You should end up with 500 mL (2 cups) of lentils with broth.
- In a skillet set over medium-high heat, heat olive oil and cook onions for 3 minutes. Add garlic, thyme, and mushrooms and cook for 8 minutes, stirring occasionally.
- Add tomato paste, soy sauce, mustard, and lentils with broth.
- Continue cooking for 5 minutes over low heat. Taste and season as needed. Set aside. Preheat oven to 200°C (400°F).
- Peel potatoes and cut into 2-cm (¾-in.) cubes.
- In a saucepan, add water and garlic, bring to a boil, and add potatoes. Cover and cook over medium heat for 15 minutes stirring occasionally.
- Mash with a pestle or potato masher. Add lemon zest and season with salt and pepper to taste.
- Pour mushroom mixture into a 20 cm x 20 cm (8 in. x 8 in.) cooking dish and cover with mashed potatoes. Even out the top and sprinkle with knobs of butter, Parmesan, and panko bread crumbs.
- Bake on middle rack for 25 to 30 minutes. Let stand for 5 minutes before serving.
- Serve with green salad if desired.
You can prepare the lentils the day before. If you refrigerate the prepared dish before cooking, bring to room temperature for 30 minutes and add 10 minutes of cooking time.