Place the sirloin strips, smashed garlic, kosher salt, pepper, lemon juice, and cumin in a plastic zip-top bag or container and marinate at room temperature for 30 minutes.

Drain and discard the liquid off of the marinated steak, remove and discard the garlic cloves and set aside. Heat 2 tablespoons of the vegetable oil in a large skillet over medium-high heat. When the oil is hot, add the onions and cook, stirring, 2 minutes. Add the reserved steak and the bell peppers and cook, stirring 2 minutes. Add the minced garlic and cook an additional 1 to 2 minutes, or until the steak is at the desired degree of doneness. Season with 1/4 teaspoon of salt and the pepper, remove from the heat and set aside.

Spread 1 tablespoon of the Chipotle Mayonnaise over each half of the panini bread. Divide the steak, onion, and pepper mixture over 4 pieces of panini bread. Top with shredded cheese and the other panini half.

Heat a panini press or grill pan over high heat. Spread 1 teaspoon of vegetable oil over the top of each panini and 1 tablespoon over the bottom of each panini. When the panini press or grill pan is hot, place the paninis onto the hot surface and cook 1 to 2 minutes, or until the cheese is melted and the sandwich is golden brown. If using a grill pan, place the sandwich onto the hot surface, place a skillet on top of the sandwich and firmly press down on the skillet to flatten the sandwich. You will need to turn the panini over and repeat the process for another 1 to 2 minutes. Serve hot.