Step 1: Boil the Potatoes. Place the chopped potatoes in a large pot of salted water. Bring to a boil and cook for about 15-20 minutes, or until the potatoes are fork-tender. Drain the water and set the potatoes aside for mashing.
Step 2: Sauté the Aromatics. While the potatoes are boiling, heat a large skillet or Dutch oven over medium heat with a splash of oil. Add the onion, carrots, and celery. Cook for 5-7 minutes until the vegetables have softened and the onions are translucent.
Step 3: Season and Simmer. Stir in the minced garlic, tomato paste, thyme, and rosemary. Cook for another minute until fragrant. Add the rinsed lentils and the vegetable broth. Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 20-25 minutes until the lentils are tender but not mushy.
Step 4: Prepare the Mashed Topping. Return to your cooked potatoes. Add the butter and milk. Mash until smooth and creamy. Season generously with salt and pepper. If you like a bit of texture, you can leave a few small lumps.
Step 5: Finish the Filling. Once the lentils are cooked, stir in the frozen peas and season the mixture with salt and pepper to taste. If the mixture looks too dry, add a splash more broth. Transfer the lentil mixture to a 9×13 inch baking dish.
Step 6: Assemble and Bake. Spread the mashed potatoes evenly over the top of the lentil filling. Use a fork to create decorative ridges on the surface; these will turn crispy and golden in the oven. Bake at 400°F (200°C) for 15-20 minutes, or until the top is lightly browned and the filling is bubbling.