Fry the kielbasa slices in a large pan til dark and crispy on both sides. 8-10 minutes at medium. Remove the kielbasa from the pan and set aside in a medium mixing bowl
Add the onions and garlic to the pan. Fry, stirring regularly til onions are brown. 8-10 minutes. Remove from pan and add to the kielbasa in the bowl. Toss to combine.
Pour the beer into the pan, bring the heat to high and, once boiling, scrape the bottom of the pan. Reduce heat to medium
Add 3 tbsp of butter and the flour, whisk til combined then add the cream, mustard, thyme and season with pepper. Simmer at medium for 3-5 minutes, whisking frequently until thickened. Taste the sauce and adjust as needed with more mustard and / or pepper.
Butter the buns and broil til toasted. Divide the kielbasa and onions among the buns and top with sauce. Serve the sandwiches cut in half- they'll be less messy.