In a small bowl, mix together brown sugar, coarse black pepper, and sea salt until well combined. Generously rub this mixture all over the eye of round beef roast, ensuring an even coating. Allow the roast to sit at room temperature for 30 minutes.

While your roast is resting, preheat your oven to 500°F (260°C).

In a large skillet, heat a drizzle of avocado oil or olive oil over medium-high heat. Once the oil is shimmering, add the seasoned roast to the pan, searing it for about 2 minutes on each side until it develops a deep brown crust.

Transfer the roasted beef to an oven-safe dish and place it in the preheated oven. Immediately turn off the oven after placing the roast inside. Roast until it reaches an internal temperature of 125°F for a perfect medium-rare, which typically takes about 1 hour.

Remove the roast from the oven and set it on a cutting board. Tent it loosely with aluminum foil and allow it to rest for 20 to 45 minutes.

With the skillet still over medium heat, add unsalted butter and finely chopped onion, sautéing until the onion is browned and fragrant. Next, add in red wine vinegar, beef broth, and soy sauce, stirring to combine.

Allow the mixture to simmer for a few minutes until it thickens slightly, then blend until smooth for a velvety sauce.

After the roast has rested, slice it against the grain into thin pieces. Arrange the juicy slices on a serving platter and generously drizzle with the warm, blended sauce.