Preheat the oven to 350°F.
Season the short ribs evenly with salt and pepper.
In an oven-safe Dutch oven, heat olive oil over medium-high heat. Once hot, sear ribs on all sides until browned. Do not overcrowd the pan, work in batches if needed. Transfer the ribs to a plate and set aside while you make the broth.
Reduce heat to medium, add onions, tomato paste, and beer. Whisk together and cook for 5 minutes, scraping brown bits from the pan as you go.
Add broth, Worcestershire sauce, and stir to combine.
Nestle ribs into the broth and add herbs.
Bring to a boil, cover, and transfer to the oven. Cook for 2.5 to 3 hours, or until the meat is fall off the bone tender.
Carefully remove the ribs and transfer them to a plate. Tent with foil to keep warm.
Discard the herbs and transfer the liquid to a large bowl.
Carefully skim off the top layer of fat. The remaining broth will be used to create a gravy. Discard the fat and set the broth aside.
To the original Dutch oven over medium heat, melt butter. Add flour and whisk together. Cook for 1 minute and then slowly pour the broth into the pot, whisking constantly.
Simmer until thickened (about 5 minutes).
Serve short ribs and gravy over mashed potatoes. Enjoy!