While pastry is resting prepare the filling
Place the onions, vinegar and sugar in a bowl, toss to combine and set aside
When ready to roll out the pastry, preheat the oven to 200°C fan forced
Break an egg into a small bowl, lightly mix with a fork and put aside
Place a large piece of baking paper on your bench (once the pastry is rolled out and filled on the baking paper, you will lift this paper with the tart on it, onto a large baking tray so get that ready too)
Remove pastry from fridge, dust the baking paper with a little flour, place pastry on top and dust that lightly with flour too
Roll pastry out to a 35cm round, dusting lightly with flour as needed (the pastry will overlap slightly on the baking paper but don’t worry as you will be folding it in)
Place sour cream, reserved 175g milled cheddar, seasoning and mint in a small bowl and mix to combine
Spread sour cream mixture over the pastry circle, leaving a 5 to 6cm border clear
Add the baked carrots to the red onion mixture and gently toss together
Leaving behind any juice in the base of the carrot/red onion bowl, place carrot and red onion mixture on top of the sour cream mix
Lightly brush the edges of the pastry with the beaten egg, then lift the edges up and fold the pastry over onto the filling leaving the middle open (the egg will help the pastry stick together)
At this point carefully lift the baking paper with the tart on it, and place onto a baking tray
Brush the top of the pastry with the egg, then pour any leftovers of the egg into the middle of the pastry so that you don’t waste it
Sprinkle the pastry border with either nigella seeds or sesame seeds
Bake in the oven for 30 minutes, turn tray around and continue for a further 10 minutes, or until a rich golden colour (it usually takes 40 minutes in my oven)
Cool for 10 minutes, before topping with fresh mint leaves and serving