Prepare the Lentils: In a medium saucepan, add the lentils and enough water to cover them. Bring to a boil, then reduce heat and simmer for 20-25 minutes until tender. Drain and set aside.

Cook the Vegetables: In a large skillet, heat the olive oil over medium heat. Add onions and carrots, cooking until softened, about 5 minutes. Add garlic, peas, tomato paste, thyme, and rosemary, stirring to combine.

Combine Lentils and Vegetables: Add the cooked lentils to the skillet with the vegetables. Pour in the vegetable broth, and bring the mixture to a simmer. Cook for 10 minutes, or until the mixture thickens. Season with salt and pepper.

Make the Mashed Potatoes: While the lentil mixture is cooking, boil the potatoes in a large pot of salted water until tender, about 15 minutes. Drain and return to the pot. Add plant-based milk and vegan butter, mashing until smooth. Season with salt and pepper.

Assemble the Pie: Preheat the oven to 375°F (190°C). Spread the lentil-vegetable mixture in the bottom of a baking dish. Top with the mashed potatoes, spreading evenly.

Bake the Pie: Bake in the preheated oven for 20-25 minutes, or until the top is golden and the edges are bubbly.

Serve and Enjoy: Let the Shepherd's Pie cool for a few minutes before serving.