Before you begin, rest and bring the sirloin roast to room temperature (about 45 minutes).
Peel the garlic cloves and place into the center of a square of foil. Drizzle with olive oil and season with a pinch of salt. Close foil and place into the oven as it preheats to 250°F.
Season steak with salt, garlic salt, pepper, oregano, and minced onion.
Place roast into a pan and into the oven on the center rack. Cook for 1 hour.
Remove the garlic and steak. Brush the steak with Dijon mustard and ½ of the roasted garlic. Sprinkle parmesan cheese generously over the top. Place the steak back into the oven for 55 minutes.
Turn off the oven and let the roast rest inside the oven as it finishes cooking and comes to temperature.
Remove the steak from the oven and place onto a wooden cutting board. Let rest 10 minutes. Slice steak and serve with mashed potatoes
Add quartered potatoes (skin on) to a large pot and fill with water until the potatoes are just covered. Bring to a boil and boil for 15 minutes.
Add milk, heavy cream, butter, and reserved roasted garlic to a saucepan over medium heat.
Remove the potatoes from the water and into a large bowl. Lightly mash.
Once the saucepan starts simmering, pour it into the bowl with the potatoes and stir until combined Season lightly with salt and pepper to taste.
Do not open the oven during roasting. Slow roasting the steak provides an incredibly tender and juicy steak. Use a meat thermometer as cooking times may vary depending on the size and weight of the steak and ambient oven temperature.
Internal temperature should be 145 degrees F for Medium, i.e., brown outsides and light pink in the middle.
Internal temperature should be 155 degrees F for Medium-Well, i.e., brown outsides and slightly pink in the middle.
Internal temperature should be 165 degrees F for Well-done, i.e., brown outsides and in the middle.