Directions :

1. In a food processor or blender, blend marinade ingredients until smooth. Reserve two tablespoons for assembly.
2. Place steak into a sealable plastic bag and pour remaining marinade on top. Press the air out of the bag, seal, and massage the marinade into the steak. Let sit in the refrigerator for at least 4 hours, up to overnight.
3. Place the onions in a container with a lid. Bring the remaining ingredients to a boil and whisk until sugar and salt are completely dissolved.
4. Pour the vinegar mixture over the onions, making sure all are submerged. If any onions are floating above the surface, use a sheet of cling wrap to keep them under. Let cool to room temperature before using or cover and store in the refrigerator until ready to use.
5. Combine the seasoning mix ingredients in a small bowl and set aside.
6. In a cast iron skillet, heat canola oil over medium high heat. Add garlic and jalapeño and cook until garlic begins to brown slightly, about 30 to 60 seconds.
7. Add pinto beans and continue to cook for 3 minutes, stirring occasionally until skins start to brown and blister. Add vegetable stock and mash vigorously with a potato masher until creamy. Add 1 teaspoon of seasoning mix and half of reserved cilantro leaves.
8. In a small bowl, combine the salsa ingredients and toss with a spoon.
9. Adjust seasoning to taste.
10. Remove the steak from the refrigerator and bring to room temperature. Heat a grill pan, cast-iron skillet or flat top over high heat. Pour a small amount of canola oil onto a wad of paper towel and rub the grill grates until they are coated with a thin layer of oil. While the pan is heating, wipe marinade from skirt steak and season with salt and freshly ground pepper.
11. When you see light wisps of smoke coming from the pan, lay the skirt steak on the grill at a 45 degree angle and cook for 2 minutes. Turn the steak counterclockwise 45 degrees and continue grilling for another minute. Flip and repeat on the other side. Internal temperature should be at least 125℉.
12. Rest for 5 minutes under a foil tent before slicing against the grain into thin strips. Lower the heat to medium-low.
13. Slice and toast the telera buns, cut side down, on the grill until golden brown.
14. While buns are toasting, in a small bowl combine 2 tablespoons of reserved marinade and mayo. Spread a generous amount on both sides of the toasted buns.
15. Build the sandwich by spreading one side of each roll with a thin layer of refried beans. Scatter with pickled red onions, then layer the sliced steak on top. Lay two slices of queso fresco on top of the steak. Feel free to crumble the queso fresco but a slice makes for a sturdier sandwich. Pile with avocado-radish salsa and leafy lettuce.
16. Secure with the top bun. Press down slightly and slice in half before serving.