Rinse lentils thoroughly. In a large pot or Dutch oven, combine rinsed lentils and vegetable broth. Bring to a boil, then reduce heat, cover, and simmer for 25-30 minutes, or until tender. Drain any excess liquid and set aside.

While lentils cook, heat olive oil in a large skillet over medium heat. Add onion, carrots, and celery. Cook, stirring occasionally, until softened, about 8-10 minutes.

Add minced garlic and tomato paste to the skillet. Cook for 1-2 minutes until fragrant, stirring constantly.

Stir in the cooked lentils, thyme, rosemary, bay leaf, and Worcestershire sauce. Season with salt and pepper to taste. Bring to a simmer, then reduce heat and cook for 5 minutes, allowing flavors to meld. Remove bay leaf. Stir in frozen peas.

Meanwhile, place cubed sweet potatoes in a pot and cover with cold water. Bring to a boil and cook for 15-20 minutes, or until fork-tender. Drain well.

Transfer drained sweet potatoes to a bowl. Add milk and butter. Mash until smooth and creamy. Season with salt and pepper to taste.

Preheat oven to 191 °C (190 °C). Pour the lentil filling into a 9x13 inch baking dish.

Carefully spread the mashed sweet potato topping evenly over the lentil filling. You can create decorative swirls with a fork if desired.

Bake for 20-25 minutes, or until the topping is lightly golden and the filling is bubbling around the edges.

Let stand for 5-10 minutes before serving.