To make the marinade, mix together all the ingredients in a plastic bag with a resalable mouth or an airtight container. Place the meat in the marinade, seal and place it in the fridge for at least 4 hours or overnight.
To make the pico de gallo, place all the ingredients in a bowl, season generously with salt and pepper and set aside.
To make the guacamole, place the red onion, avocado, jalapeno, tomatoes, garlic, and limejuice in a bowl. Season with cumin, salt and pepper. Give it a gently mix and set it aside.
When you are ready to serve, heat your grill until it is very hot, 10-15 minutes. Remove the meat from the plastic bag and discard the marinade. Place it on the grill grate and grill until well-seared and dark brown on one side, 4-5 minutes. Using thongs, flip the steak and continue to grill until the interior meat is slightly less done than you want it to be when you eat it, 3-4 minutes for medium or 2-3 for medium-rare. Transfer the meat onto a cutting board, loosely cover with aluminum foil, and let it rest for 7-10 minutes. Slice it on bias against the grain.
While the meat is resting, warm the corn tortillas on the grill until they are softened, 1 minute or so on each side.
When ready to assemble your tacos, place a few slices of meat at the bottom of a corn tortilla and top it off with a few tablespoons of guacamole and pico de gallo.
Garnish it with sliced cucumbers and cilantro leaves. Repeat the same process with the rest of the tacos and serve.