Chop the top and the root of the onions off. Cut the onions in half and remove the skin. Slice the onions in thin slices.
In a casserole, melt some butter on a medium high heat. Put in the sliced onions and stir to coat all the onions with butter. Slowly let the onions glaze, turn translucent and eventually turn brown and caramelize. When the onions start to caramelize and turn brown, add the sugar. Season with salt and pepper to taste. Once the onions are caramelized, turn off the heat and transfer the onions to a bowl and set aside.
In the same casserole add a little butter and melt over a medium heat, add the fennel seeds and the sausages. Cook the sausages turning them every few minutes till they're nice golden brown on all sides. Keep turning them for roughly 12 minutes. each sausage is different so yours might take a little longer or a little less time to cook Make sure they're really hot and thoroughly cooked through and that no pink remains.
When the sausages are done, turn off the heat.