Sauce):
Freeze the steaks until firm, 25 to 30 minutes. Meanwhile, in a blender, combine the lime, orange and grapefruit juices, achiote paste, vinegar, 1 teaspoon kosher salt (or ½ teaspoon fine sea salt), garlic, oregano, black pepper and cumin. Blend until smooth. Transfer to a large bowl or resealable container. Now’s a good time to make the chiltomate salsa:
— Heat a large cast-iron skillet or griddle to high and turn on your fan, if you have one (or open some windows). Cook the tomatoes and habanero, turning every few minutes, until blackened all over and the tomatoes are soft and bursting, 10 to 15 minutes for habanero and 25 to 30 minutes for the tomatoes. Set aside on a cutting board to cool slightly. Scrape all the blackened bits from the skillet and transfer to a mortar or large bowl.
— Chop the tomatoes and transfer to the mortar or large bowl. Using gloved hands if desired, remove and discard the stem and seeds from the habanero and finely chop.
Transfer half the habanero to the mortar.
Finely chop all the cilantro, including the stems, and add to the mortar. Season with salt and mash into a chunky sauce. (If using a bowl, squeeze with gloved hands.) Season to taste with salt and lime juice; if tomatoes are at peak ripeness, lime juice may not be necessary. Add more habanero to taste.
Salsa keeps covered and refrigerated for 5 days. Bring to room temperature before serving.
Cut the steak in half crosswise and lengthwise, then slice in thirds through the middle (as if you’re cutting open a burger bun). Repeat with the remaining pieces until you have 12 flat, thin pieces. Transfer to the marinade, cover and refrigerate for 4 to 24 hours, stirring occasionally.
When ready to cook, start by warming the tortillas. Lightly sprinkle the tortillas with water. Lightly grease a large cast-iron skillet or griddle and set over high heat. Working in batches, add one layer of tortillas to the skillet and warm for 30 seconds per side, then another 15 seconds more on the first side. Transfer to a clean kitchen towel, cover, and repeat with the remaining tortillas. Set aside while you cook the steak; reserve the skillet.
Drain the steak, discarding the marinade, and toss the steak with the oil. Sprinkle with salt. Heat the same skillet or griddle over high. Working in batches, sear the steak until charred and unstuck from the skillet, about 1 ½ minutes per side.
Build the tacos by stacking two tortillas, then a small handful of lettuce, one piece of steak and a spoonful of salsa. Use a spoon to scoop a thin sliver of avocado, then add to the tacos, followed by a sprig of cilantro. Serve with lime wedges for squeezing over.
