To make the sauce, in a small bowl, combine the mayo, sour cream and horseradish until smooth and evenly combined. Set to the side.

Heat a cast iron skillet over medium high heat with the olive oil and butter. Season the steak with salt and pepper on both sides. Place the steak in and cook for about 5 to 7 minutes or until crispy and browned and then flip over and continue to cook for another 3 to 4 minutes on the second side. Transfer to a plate or platter and allow to rest before slicing.

Add the onions to the hot skillet, right where the meat was cooked. Season with a pinch of salt and pepper and sauté until soft and caramelized.

To assemble the sliders, cut the ciabatta loaf into 8 to 10 small squares. Split down the middle and spread each half with the creamy horseradish sauce. Layer with slices of cheese, the sliced steak and caramelized onions. Sandwich the panini halves together and drizzle the tops with olive oil.

Heat a panini press and then place the sliders on the press (a few at a time) and cook until the bread has toasted and become golden brown and crunchy and the cheese has melted. Continue with the rest. Serve warm or at room temperature. Enjoy!