Peel the sweet potatoes and Maris Pipers, then chop into 2cm chunks. Parboil in a large pan of boiling salted water for 5 minutes, then drain and allow to steam dry. Peel and finely slice the onions.
Heat 1 tablespoon of oil in a large frying pan over a medium heat, add the onion and cook for 5 minutes, or until starting to soften, stirring regularly.
Pick in the thyme leaves, season with black pepper, then add
all the potatoes and cook for 15 to 20 minutes, or until golden and crispy, squashing and turning regularly with a fish slice.
Deseed and finely slice the chilli (if using) and scatter over the hash, then serve straight away.