Season the pork cubes with salt and pepper. Melt 2 tablespoons of butter in a Dutch oven over medium heat and brown the pork in batches until all sides are seared. Remove the pork and set it aside.
Add the remaining 2 tablespoons of butter to the pot. Add the sliced onions and cook over medium-low heat, stirring often, until they turn golden brown and soft, about 25 minutes.
Stir in the minced garlic and cook for 1 minute. Sprinkle the flour over the onions and stir for 2 minutes to form a light roux.
Pour in the red wine and scrape the bottom of the pot to loosen any browned bits. Let the wine simmer for 3 minutes.
Return the pork to the pot. Add the beef broth, thyme sprigs, and bay leaf. Bring to a gentle boil, then reduce to low heat, cover, and simmer for 1 hour and 15 minutes until the pork is tender and the gravy has thickened.
Remove the thyme sprigs and bay leaf. Taste and adjust salt and pepper if needed. Serve hot with slices of crusty bread on the side.