Step 1: Cook the Potatoes : Place potatoes in a pot of salted water and bring to a boil. Cook until tender, about 12–15 minutes. Drain, mash with butter and milk, and season with salt and pepper. Set aside.
Step 2: Prepare the Lentil Filling : Heat olive oil in a large skillet over medium heat. Add onion, carrots, celery, and mushrooms. Cook for 5–7 minutes until vegetables soften. Add garlic and cook for 30 seconds.
Step 3: Add Lentils and Sauce : Stir in lentils, vegetable broth, tomato sauce, tomato paste, thyme, rosemary, smoked paprika, salt, and pepper. Bring to a simmer and cook for 20–25 minutes until lentils are tender and the mixture thickens.
Step 4: Add Peas and Corn : Stir in frozen peas and corn. Cook for another 2 minutes. Taste and adjust seasoning as needed.
Step 5: Assemble the Shepherd’s Pie : Preheat the oven to 400°F (200°C). Spread the lentil filling evenly in a baking dish. Top with mashed potatoes, smoothing them over the filling. Use a fork to create texture on the surface.
Step 6: Bake : Bake for 20–25 minutes or until the top is lightly golden and the filling is bubbling around the edges. Allow it to rest for 5 minutes before serving.