1 Set the oven to 200 degrees.
2 Put the butter, water, onions, thyme and cinnamon into a pan. Cover and cook gently for 15 minutes until the onions soften.
3 Remove the lid and turn up the heat so the liquid starts to evaporate and the onions start to caramelise. Scrape the bottom of the pan periodically, making sure you dislodge any sticky brown bits as they appear.
4 Add the sugar. This will speed up the caramelisation process, so keep a good eye out so it doesn't start to burn.
5 After a few minutes, when the onions are lightly golden, add the red wine vinegar, maple syrup, salt and pepper, then transfer the onions on to a baking tray.
6 Smooth the onions out a little, then place the sausages on top and bake for 15 minutes.
7 Tear the croissants into rough chunks, then nestle them in among the sausages. Bake for another five minutes, then eat with mustard.