Sweet Potato, Red Onion and Lentil Quesadillas

medium sweet potatoes (~500g total, peeled)

olive oil

red onions (sliced 5mm thick)

salt

balsamic vinegar

green lentils, drained

ground cumin

chilli flakes (or more/less if you want)

juice of ½ a lemon

large handful of coriander (roughly chopped)

cheddar cheese (grated)

large wholemeal tortillas

Greek yogurt (to serve)