Preheat oven to 200°C.
Boil potatoes in salted water for 15 minutes, then add peas and cook until tender.
Sauté onion, garlic, carrots, and celery in olive oil until soft.
Add flavorings and liquids to the vegetables, simmer for 10 minutes until sauce reduces, then stir in lentils.
Fry spring onions in melted butter, then add milk and heat through.
Mash potatoes and peas with the warm milk mixture until chunky.
Transfer lentil mixture to a pie dish, top with mash, then scatter cheese and paprika.
Bake for 25 minutes until the topping is golden-brown.