Classic French Dip Recipe

2 pounds Certified Angus Beef top sirloin roast
1 Certified Angus Beef shank (about 1 pound)
3 teaspoons coarse kosher salt, divided
1 teaspoon fresh cracked pepper
1 tablespoon neutral oil (like grapeseed or canola)
1 large yellow onion, root removed and cut in half
1 quart salt free beef broth
6 slices Swiss or provolone cheese (optional)
1/4 cup hot mustard
1 (24-inch) French loaf or four 6-inch French rolls