Make the Sriracha mayo by mixing all the ingredients with a small whisk. Set aside.
Cut the sirloin steak into 1 1/2 inch cubes. Trim away excess fat and tough connective tissue.
Put the steak cubes in a medium mixing bowl, drizzle with olive oil, and sprinkle with Montreal steak seasoning.
Mix the meat and seasoning with your hands, making sure all the steak chunks are thoroughly coated. Let the steak marinate for about 20 minutes.
Meanwhile, preheat a 12-inch cast iron skillet over medium high heat. Let the skillet preheat for 5-10 minutes.
Put the steak pieces in the skillet, making sure not to crowd the pan. If needed, you can cook the steak bites in two or more batches.
Once the steak bites are in the pan, add the butter. Tilt the pan if necessary to distribute the butter.
Let the steak bites brown thoroughly on one side, for about 2 minutes. Flip the steak bites with tongs, and brown on the other side.
Cook the steak bites to an internal temperature of 130 degrees for medium rare, or 135 for medium, measured with an instant read cooking thermometer.
Remove steak bites from heat and serve with Sriracha mayo as a dipping sauce.