Combine all marinade ingredients in a bowl. Whisk until blended. You want the brown sugar to dissolve completely, allowing flavors to meld. Pat the sirloin dry and place it in a resealable plastic bag. Pour the marinade over the steak, seal it, and refrigerate for at least 2 hours or overnight for best results.

Once you're ready to cook, take the steak out of the marinade. Let it sit at room temperature for about 30 minutes. This helps it cook evenly.

Heat your skillet or grill over medium-high heat. Add butter and olive oil until hot. This combination results in a rich, flavorful cooking base.

Place the steak on the hot surface. Don't crowd the pan. Season with sea salt, black pepper, and brown sugar. The sugar will caramelize, creating a lovely crust. Sear for about 4-5 minutes on each side for medium-rare, depending on steak thickness.

In another pan, you can cook the eggs simultaneously. Use the same heat, and cook to your liking. I prefer sunny-side-up, as the yolk adds moisture.

Once the steak is done, let it rest for a few minutes. This rest allows the juices to redistribute. Slice against the grain before plating it alongside eggs. Top with sautéed onions if desired.