Preheat oven to 425F/220C. Dice onion and garlic, and chop fennel, celery, carrots, and potatoes. Add coconut oil to saucepan and bring to medium-high heat. Add onions and garlic, season with salt and pepper and cook for 15 mins, stirring occasionally until veg becomes translucent in colour.
Add fennel, carrots, celery, potatoes and thyme. Season with more sea salt and pepper and let veg sweat for 15 mins, stirring occasionally. Then add lentils and vegetable stock. Bring to a simmer and cook until thick and soupy (20 mins).
Lay sheet of pie pastry on flat surface and cut 4 circles to be 1-inch wider than ramekins. (To do this turn ramekin face down onto pastry and lightly trace around them with a knife to be one inch wider).
Pour brothy veg into ramekins and top each with puff pastry. Pinch the edges of the pastry with your forefinger and thumb, working your way entirely around the edge of the pies for a textured effect. Lightly paint pastry with olive oil and cut three small slits in the centre of each pie.
Transfer mini pot pies to oven to bake for 30 minutes until golden brown, then reduce heat to 350F (180C) and bake for another 30 minutes.