In a large pan toss in the beetroot, carrot, onion, garlic, olive oil herbs and spices. Gently cook until tender then stir in the lentils and lemon juice. Simmer gently for a couple of minutes and stir to combine then leave to cool completely. Season with salt and pepper.
Roll out the pastry into a fat oblong. You might prefer to make smaller wellingtons, make the sizes that will fit on your baking sheet and are usable for you. But wide enough to take a long log of the beetroot mixture.
Down the centre of the pastry pile the beetroot mixture into a long log. Brush all the borders with milk, fold over the long sides, then carefully completely seal in the ends to encased the filling into a roll. Place your Wellingtons on a baking tray lined with baking parchment and chill for at least 1 hr, or cover with cling film and leave overnight. If freezing, cover and freeze on a lined baking tray for up to 3 months.
To be honest I often don’t have time to chill them and wait so I regularly bake them off straight away.
Brush the top of each Wellington with milk then bake for 30 mins from chilled or 45 mins from frozen until crisp and golden.
To bake from chilled, heat oven to 190 C/170 C fan/gas 3 and line a baking tray with parchment.
Scatter over extra thyme and some flaky sea salt and serve.