Warm a large wide pot on a medium heat, add the oil and onions, cook to soften for 5-10 minutes. Add the garlic and carrots, celery if using, cook for a further few minutes. Next add the paprika, cumin, herbs, tomato puree, honey, salt and pepper, stir to coat the veg in the spices.
Go ahead and add the walnuts or mushrooms, if adding mushrooms cook them until they shrink down. Then add the washed lentils, chopped tomatoes, stock and cook and simmer for 30-40 minutes, lid off. Stir occasionally. The sauce should reduce and thicken. Once the veg is cooked and the lentils are soft it's ready. Taste, and adjust the seasoning, add tamari and a dash of balsamic vinegar if it needs it.
In the meantime cook the potatoes, bring a big pot of water to the boil and add salt. Add the potatoes and cook until soft, about 15 minutes. Pierce with a sharp knife. Drain the water, mash with the butter. If using cheese add half when mashing.
Assemble the pies, add the lentils to the dish top with the mash, sprinkle over cheese if using.
If cooking straight away: Preheat the grill. Pop the pies under the grill until golden, keep a close eye.
To cook from cold: Preheat the oven 180ºc fan, heat through for 30-40 minutes, and grill for a few minutes to get a crunchy top.
Storage: The full pie is good in the fridge for 3 days.
Freezer: Wrap the pie in food safe cling film or a food bag. Store in the freezer for 3 months.
Defrost: Defrost overnight in the fridge.