Finely dice the onion and celery and put in a medium saucepan with a little olive oil. Cook over a medium heat for around 5 minutes until the vegetables soften
Mince the garlic, add to the pan and cook for a further minute
Peel and dice the beetroot and add to the pan along with the stock and a good grind of black pepper. Don't be tempted to add salt yet - the feta and capers are salty
Cover with a lid and let cook for 10-15 minutes until the beetroot is fork tender
Add the lentils and continue to cook on a high heat until most of the liquid has evaporated, then take off the heat
Rinse the capers in fresh water and mince roughly. Add to the pan along with the oregano
Crumble the feta into the pan and give everything a good stir. Taste and add more salt/ pepper as desired
Spoon into 1 large pie dish or 4 individual dishes