Preheat the oven to 200C/400F/Gas 6. For the champ topping, heat a pan of salted water, add the potatoes and bring gently to the boil, then simmer until the potatoes are nearly cooked through, about 15-20 minutes. When the potatoes for the champ topping are almost done, add the frozen peas and cook for a few more minutes until the peas are tender. Drain and set aside.
Meanwhile, for the lentil mixture, heat the olive oil in a large frying pan over a medium heat and gently fry the chopped onion, garlic, carrots and celery for 5-10 minutes, until softened.
Add the remaining lentil mixture ingredients except for the lentils and season to taste with salt and freshly ground black pepper. Bring to the boil then reduce the heat and simmer for 10 minutes until the vegetables are tender and the sauce has reduced. Stir the lentils into the tomato sauce.
Melt the butter in a small saucepan over a low heat, then gently fry the spring onions until softened. Add the milk and heat through.
Mash the potatoes and peas roughly. Add the warm milk mixture to the potatoes and continue to mash until combined but still chunky.
Spoon the lentil mixture into a medium pie dish and top with the champ. Scatter over the cheese and smoked paprika.Place the dish onto a baking sheet and bake the pie for 25-30 minutes, or until the potato is golden-brown.