VEGETARIAN SAUSAGE ROLLS

1 medium-large sweet potato (about 250g) • 200g mixed nuts • 1 tsp olive oil • 1 large red onion, finely diced • 3 tbsp red wine or port • 1 tsp picked thyme leaves • 2 tsp Dijon mustard • 50g vegetarian cheddar, grated • 1 medium British Blacktail Free Range Egg, beaten • 320g Jus-Rol Puff Pastry Sheet • 1 tsp nigella seeds