Place your frypan over medium heat. Place sausages in pan, cook on all sides until nice and golden. Remove to a plate. It doesn't matter at this stage if the sausages are cooked all the way through, they will cook further in the sauce later on. Leave any sausage fat in the pan.
Add 3 tsp of oil into the frypan, still over medium heat. Add your sliced onions and cook them for about 3-4 minutes. Add garlic, cook for a further 2 mins.
Now you have to do a bit of looking at your situation and adding more or less oil, depending on your situation. To start the roux we need about 3 tsp of oil in the pan. Depending on how fatty your sausages were, and what is left after sautéing your onions, will depend on how much oil you need to add. So add what oil you need to, to make 3tsp of oil.
Add the flour to the oil and onions, and stir well, cook the flour out for about 1 minute, before adding all of the beef stock at once. As soon as the stock hits the pan, stir vigorously for about 2 minutes, to make sure there is no lumps. Add in all the herbs and seasonings, and return the sausages to the pan. Once it starts to bubble turn down to medium-low.
Simmer sausages uncovered for 10 minutes, and sauce is thickened. Serve with buttery mashed potatoes, and steamed carrots and peas.