1. MUSHROOM LENTILS: Rinse and drain the lentils. Bring the broth to a boil and add the lentils. Cook over low heat, partially covered, for about 30 minutes or until tender. Season with salt and pepper.
2. In a large skillet, heat the olive oil over medium-high heat and cook the onions for 3 minutes.
3. Add the garlic, thyme, and mushrooms. Cook for 8 minutes, stirring occasionally (if using diced bell peppers, cook until they are cooked).
4. Add the tomato paste, soy sauce, mustard, spinach, and lentils with their juice. Continue cooking for 5 minutes over low heat.
5. Taste and adjust the seasoning if necessary.
6. Set aside.
7. SWEET POTATO PUREE: Put the water and whole garlic clove in a saucepan, bring to a boil, and add the sweet potatoes. Cover and cook over medium heat for 15 minutes, stirring occasionally.
8. Mash with a masher or a potato ricer.
9. Add the lime zest and 1 tablespoon of butter. Season with salt and pepper to taste.
10. Preheat the oven to 200 °C (400 °F).
11. Pour the lentil and mushroom mixture into a 20 cm x 20 cm (8 in x 8 in) baking dish and top with the sweet potato puree. Smooth the top and sprinkle with 1 tablespoon of butter, Parmesan cheese, and panko breadcrumbs, if desired.
12. Bake in the center of the oven for 25 to 30 minutes.
13. Let it rest for 5 minutes before serving.