Add beef broth, pepperoncini juice, pepperoncini peppers, onion, Italian seasoning, garlic powder, onion powder, sugar, salt, black pepper, and yellow mustard to the crock pot. Stir gently.
3 cups beef broth, ½ cup juice from a jar of pepperoncini peppers, 6-8 pepperoncini peppers, 1 large onion, 1 Tablespoon Italian seasoning, 1 teaspoon garlic powder, 1 teaspoon onion powder, 1 teaspoon sugar, 1 teaspoon salt, ½ teaspoon black pepper, 1 teaspoon yellow mustard