Pepito: Steak & Avocado Sandwich

1 1/2 pounds flank steak
3 tablespoons soy sauce
1 teaspoon dijon mustard
3 garlic cloves finely chopped or pressed
1 teaspoon dried rosemary
1/8 teaspoon black pepper
Pinch kosher or sea salt
2 tablespoons olive oil
4 teleras bolillos, petite baguettes, or baguettes sliced into 3- to 4-inch pieces and cut in half
6 ounces Monterey jack cheese muenster or mild cheddar
1 cup gucamole see below
1 cup refried beans store bought or homemade

2 ripe Mexican avocados halved, pit removed, meat scooped out and mashed
3 scallions rinsed, tops removed, white and light green parts thinly sliced (about 2 tablespoons)
2 tablespoons coarsely chopped cilantro leaves optional
3 tablespoons jalapeño or serrano chile more or less to taste, minced (seeding is optional)
2 tablespoons lime juice freshly squeezed
Kosher or sea salt to taste

Marinate the flank steak with the soy sauce, olive oil, Dijon mustard, garlic, rosemary and black pepper. You may marinate it anywhere from 1/2 hour to overnight in the refrigerator. Remove the meat from the refrigerator and sprinkle with salt when you are ready to cook it.
Preheat the grill pan or grill at medium-high heat. Once it is hot, place the meat and let it cook anywhere from 4 to 5 minutes per side, depending on how well done you want the meat. You can drizzle any extra marinade right over the top of the meat while it cooks. For medium, its closer to 4 minutes per side, for over medium, closer to 5 minutes per side. Remove the meat from the heat and place it on a cutting board. Let it rest for 5 to 10 minutes, slightly covered. Thinly slice across the grain.
Preheat the oven to 350°F degrees.
Slice the baguettes, teleras or bolillos in half lengthwise and place in a baking sheet. Spread about 3 tablespoons of refried beans on the bottom half of each bread. Cover with about 3 to 4 tablespoons shredded cheese. Place in the oven and let the bread crisp and the cheese melt, for about 5 to 6 minutes. Remove from the oven.
Top the pepitos with a generous amount of the thinly sliced meat and 3 to 4 tablespoons of the guacamole. Place the tops on top! Eat while hot.

Gently mix guacamole ingredients in a bowl or molcajete and serve. It can be prepared up to 12 hours in advance if covered and stored in the refrigerator.