Melt butter and olive oil over medium-high heat

Generously salt and pepper short ribs (measurements above are guidelines and don't have to be exact)

Sear short ribs on all sides - working in batches so not to over crowd the pan - 2-3 minutes per side

Transfer short ribs to slow cooker

Pour 1/4 cup of pumpkin ale into the pan used to cook the short ribs and pour the rest of the beer over the meat in the slow cooker

Cook short ribs on low for 6 hours

Add sliced onions to pan with the beer and cook on medium for 20 - 30 minutes or until softened and caramelized

Reserve some of the beer caramelized onions for toppings

Combine remaining beer caramelized onions, horseradish, and mayonaise to the bowl of a food processor and pulse to combine to desired consistency

Remove meat from the bones and shred by pulling it apart with a couple of forks

Pile shredded short ribs onto slider rolls, top with caramelized onion horseradish sauce and finish with reserved beer caramelized onions