Bring a medium pot of water to the boil. Cut the sweet potatoes into medium chunks and add to the pot. Bring back up to the boil then leave to simmer for 10 minutes. Once cooked, drain and return to the pot.
Meanwhile cook the onions: heat the olive oil in a large non-stick frying pan over a high heat. Add the sliced onions to the frying pan of hot oil, season with salt and turn the heat down to low. Cook, stirring often, for about 10 minutes until the onions have softened. Splash in ~60ml of water and keep cooking and stirring until that water has evaporated. Repeat with ~60ml of water again. Once the water has cooked off, stir in the balsamic vinegar and cook until evaporated. Remove the onions to a small bowl. Wipe out the frying pan as you’ll use it for frying the quesadillas soon.
Add the drained lentils to the pot with the cooked sweet potato. Add generous pinch of salt, the ground cumin, chilli flakes, lemon juice and chopped coriander. Roughly mash and stir together to get a slightly chunky mixture.
Take one tortilla, spread some of the lentil/sweet potato mixture over one half of the tortilla. Top with some of the caramelised onions and some of the grated cheese. Fold in half and place into the dry frying pan set over a high heat. Cook until golden underneath then flip and cook until the other side is golden brown. Remove from the pan and cut into 3 wedges. Repeat with the remaining tortillas. Serve hot with yogurt for dipping, if desired.