Season the short ribs with chili powder, smoked paprika, salt, and black pepper. Heat olive oil in a Dutch oven over medium heat and brown the ribs on all sides. Add the diced onion and minced garlic, then pour in the beef broth. Cover and braise in a 325°F oven for about 2 hours until the meat is very tender. Remove the ribs, shred the meat with two forks, and mix it back into the reduced pan juices. Toast the baguette slices on a baking sheet until golden. Stir the sour cream and horseradish together in a bowl. Spread the cream on each toast, top with a spoonful of shredded short rib, and serve right away.