Heat oven to 200C/180C fan/gas 6. Quarter the onions, leaving the root intact so they don’t fall apart during cooking. Heat a griddle pan to high. Griddle the onions to char them, then remove and set aside. Repeat with the sausages – you don’t need to cook them through at this point, just brown them.
Tip the lentils into a roasting tin and add any juices from the griddle pan, the vinegar and some seasoning, then toss together. Arrange the onion wedges and sausages on top. Brush the sausages and onions with the maple syrup, and season the onions. Cook for 20 mins, then serve.