Boil potatoes: Place potatoes in a pot with salted water, boil for 15-20 mins or until fork-tender. Drain and mash with butter, milk, salt & pepper.
Cook veggies: Heat oil in a skillet, sauté onion, carrot, and celery until soft. Add mushrooms (if using) and cook until browned. Stir in garlic for 1 min.
Flavor base: Stir in tomato paste, thyme, and paprika; cook until fragrant
Build filling: Add lentils, broth, and soy sauce. Bring to a simmer. Sprinkle in flour (or stir in cornstarch slurry). Cook until thickened. Stir in peas. Season with salt & pepper
Assemble: Spread filling in a baking dish. Top with mashed potatoes, smoothing or roughing the top with a fork.
Bake: Bake at 400°F until golden on top and bubbling around edges. For extra crisp, broil 2 min at the end.