Dredge ribs in 3 tbs of the flour. Reserve leftover flour.
Preheat Instant Pot on Sauté high/more setting; add oil.
Brown ribs in 2 batches on all sides. Remove from the pot.
Add garlic to the pot and heat until fragrant, about 30 setons. Stir in tomato paste, marjoram, thyme and bay leaf. Sauté until tomato paste darkens in color, about 1 minute, then stir in reserved flour from dredging.
Deglaze the pot with 1/2 cup of the port, scraping up any browned bits then cook until port is reduced by 1/3 then stir in the stock, remaining 1/2 cup of port, steak sauce, brown sugar and Worcestershire sauce.
Return ribs to pot and add the carrots and onion. Close and seal the pot and set on Manual or Pressure Cook for 45 minutes.
Do a quick release on the pot and transfer the ribs to a plate. Cover with foil and let them rest for 10 minutes. Remove carrots to a serving bowl.
Mix the softened butter and the remaining 2 tbs of flour together to form a paste (this is called beurre manie) and whisk this into the liquid to thicken. Stir in the wine vinegar
Garnish with parsley if desired.
UK/EU--steak sauce is A-1 Steak Sauce. HP sauce or brown sauce is a good substitute.