In a skillet, over medium heat add olive. Add in the sausages to brown, around 6 minutes. Remove sausage and set aside.
To the skillet, add in butter. Stir in onions, simmer about 15 minutes to soften and carmelize onion, stirring frequently. Mix in minced garlic, stir a few seconds.
Pour in Guinness and stir up and brown bits sticking to the bottom of the skillet. Remove a few Tablespoons of the warm Guinness and place in a small bowl. Whisk the flour into the small bowl with the Guinness until smooth, no lumps. Pour back into the pan and whisk all together.
To the skillet, add beef broth, Worcestershire sauce, mustard, parsley, and thyme. Stir together. Add bangers back into the skillet.
Reduce heat to a low simmer. Cover pot with lid and simmer for 20 minutes. Serve over mashed potatoes.
Optional: garnish with fresh chopped parsley.