Step 1: Steam the cubed sweet potatoes for 15-20 minutes until tender. Mash them with a splash of olive oil and salt, then set aside.
Step 2: In a large pot, heat olive oil over medium heat. Sauté the onion, carrots, and celery for 8-10 minutes until soft.
Step 3: Stir in the garlic, thyme, and rosemary, and cook for another minute until fragrant.
Step 4: Add the tomato paste and lentils, stirring well. Pour in the vegetable broth and bring to a boil.
Step 5: Reduce heat and simmer for 25-30 minutes, or until lentils are tender and most of the liquid has been absorbed. Stir in the frozen peas during the last 2 minutes.
Step 6: Preheat your oven to 400°F (200°C). Transfer the lentil mixture to a 9x13 inch baking dish.
Step 7: Spread the mashed sweet potatoes evenly over the lentils. Use a fork to create decorative ridges on top.
Step 8: Bake for 15-20 minutes until the top is slightly golden and the filling is bubbling. Let sit for 5 minutes before serving.