Heat a heavy-bottomed skillet over medium-high heat.
Toss together the steak, melted butter and steak seasoning in a bowl. Add to the skillet and cook until nicely seared on all sides, 4 to 5 minutes total. Remove the steak bites to a plate to rest and return the skillet to the heat.
Lower the heat to medium and add the remaining 2 tablespoons butter to the skillet. Stir in the rosemary, garlic and a pinch of salt. Cook for 1 minute, until the garlic is heated through and no longer raw. Add the cream and bring it to the boil. Reduce the heat to low and simmer, then stir in the blue cheese, horseradish, 1 tablespoon of the chives and black pepper to taste. Transfer to a serving bowl.
Place the steak bites on a platter with toothpicks, with the sauce on the side. Garnish the plater with the remaining chives before serving.