Place the potatoes in a large pot of boiling water for 15 minutes, or until fork tender.
While the potatoes are boiling, add the butter, garlic, black pepper, steak seasoning, kosher salt and rosemary to a small skillet on the stove over medium heat until the butter is melted.
Once the butter is melted, turn the temperature down to low and allow the flavors to combine for 5-10 minutes.
Once the potatoes are fork tender, drain the potatoes then return them back to the pot over low heat. Use an electric hand mixer to mash the potatoes, then hand mix in the garlic rosemary butter and half of the blue cheese.
Spoon the mashed potatoes into an oven safe bowl or casserole dish and sprinkle the remaining blue cheese on top.