Preheat the oven to 375 F. Trim the round roast of excess fat or skin. Rub generously with sea salt and pepper. Let rest until the meat is room temperature. Pound the meat into an even thickness - about 2-3 inches thick from end to end.
In a small bowl, whisk together the whole grain mustard, honey, horseradish, sherry, marjoram, turmeric and saffron threads. Transfer about half of the sauce to a separate bowl for basting (do not cross contaminate).
Brush the meat with the sauce on both sides. Arrange the onion slices and minced garlic in the center of the roast. Use the cooking twine to fold the meat around the onions in the middle, tying tightly so that the beef is a consistent roll from one end to the other.
Roast the beef for 30-40 minutes (20 minutes per pound) or until the internal temperature reaches 150 F. Remove the roast from the oven and brush on the remaining sauce.
Turn the oven on to a low broil. Broil the beef until the sauce on the outside is bubbling and browned, forming a thin crust on the outside of the beef. Remove from the oven after 2-3 minutes, when the roast is golden brown.
Let the beef rest for 20-30 minutes prior to cutting. Serve with smashed potatoes and enjoy.