Preheat oven to 325 degrees.
Let the short ribs sit out for 10 minutes before cooking. Pat dry and sprinkle with 2 teaspoons salt on all sides.
Heat a large dutch oven with a lid or similar pot to a medium-high heat. Once the pan is hot, add 1 tablespoon oil. Add the short ribs, making sure to not crowd the pan so they can brown. Brown the ribs on all sides, it should take about 15 minutes. If needed, work in batches.
Once all the short ribs are brown, remove them from the pan and set aside. Reduce the heat to medium and add the onions to the empty pot. Toss the onion in the residual fat. Saute for 5-6 minutes until slightly softened. Pour in the white wine. Use a wooden spoon to scrape up the brown bits at the bottom of the pan. Bring the wine up to a boil and reduce to a simmer. Simmer until reduced by half. Pour 1 cup of beef broth over the onions. Stir to combine. Return the short ribs along with any accumulated juices back in the pot. Cover the pot with a lid. Place in the oven and braise until the meat is tender, about 2-3 hours. Check after two hours.
Prep the bread and cheese while the ribs cook. About 20 minutes before the ribs are done: Shred the cheese. Slice baguette into 8 pieces and split horizontally. Place cut-side up on a sheet pan. Drizzle the tops with remaining olive oil and season lightly with salt and pepper. Toast at 325°F for 6–7 minutes.Set aside.
When the ribs are finished, carefully remove them from the pot and keep warm. Discard the bay leaf and thyme. Use a slotted spoon to remove most of the onions. Increase the heat on the oven to 400 degrees.
Use two forks to shred the short rib meat. Discard the bones and any fat. Divide shredded short rib over the bottom halves of the bread. Top evenly with Gruyère.Bake at 400°F for 5–6 minutes, until melted and gooey.
Make the jus while the sandwiches bake (one container, fewer steps). Place a mesh strainer over a large liquid measuring cup. Pour the braising liquid and remaining onions through the strainer. Press the onions firmly to extract as much liquid as possible. If you want a thicker jus, return the strained liquid to the Dutch oven. In the same measuring cup, whisk together: remaining beef broth and cornstarch. Slowly whisk this mixture into the pot.Bring to a boil, then reduce to a simmer until lightly thickened. You can always skip this step and simply serve the residual jus.
Spread mayo on the top bread halves if desired. Close the sandwiches. Serve immediately with the hot jus for dipping.