Combine soy sauce, cumin, chili & garlic powders, and nutmeg in a bowl. Rub over flank steak. Place in a slow-cooker. Add broth. Cook on low for 6 hours.
Simmer marinara or tomato sauce. Add lime juice and keep warm.
Assembly: Spread butter on the sides of bread that will face the panini grill. Layer with cheese, beef, avocado, onions, and Hatch chiles. Top with second piece of bread. Press down and cook until done.
Slice and serve with sauce.