In a large heavy duty ziploc bag, shake the spice ingredients together.
Add the meat and rub, roll and shake the bag to make sure all sides are covered. Seal the bag and let the flavors mingle over night or up to 48 hours if possible.
When ready to cook, preheat the oven to 425.
Place roast on a wire rack set in a roasting pan.
Roast for 10 minutes, then without opening the oven door, reduce heat to 350 and cook for about 45 minutes longer, until internal temp registers at 115
Remove the roast from the oven, cover, and let rest for at least 20 minutes.
You could either slice your meat now, or wrap it tightly in aluminum foil after it has cooled, place it in the freezer for about 30 minutes, then use a meat slicer to achieve extra thin slices. Store the sliced meat in a resealable ziploc bag.
To make the sandwiches:Preheat a griddle or panini press.
Slice the bread into ¼ inch slices and slather with garlic mustard sauce, a heaping portion of sliced onions, sliced beef, and finally top with Havarti Dill cheese before placing the bread on top.
Press and cook on panini press until cheese has melted sandwich is heated through.