Make the sauce: Position racks in the upper and lower thirds of the oven and preheat to 450 degrees F. Cut off the top of each head of garlic, then wrap individually in foil. Toss the tomatoes on a rimmed baking sheet with 1 tablespoon olive oil and a pinch of salt. Toss the onions, poblanos and the remaining 1 tablespoon olive oil on another baking sheet. Roast the garlic, tomatoes, onions and poblanos, switching the pans halfway through, until the vegetables have softened and are browned in spots, about 45 minutes. (Remove any vegetables early if they?re getting too charred.)
Squeeze the garlic cloves out of their skins. Remove the skin and seeds from the poblanos; set aside the poblanos and half each of the roasted garlic and onions for the albondigas. Puree the tomatoes, the remaining garlic and onions and the chopped chipotles in a blender, scraping down the sides as needed.
Reduce the oven temperature to 400 degrees F. Melt the lard in a medium saucepan over medium-high heat. Add the roasted vegetable puree and cook until reduced by about half, 8 to 10 minutes. Pour in the stock, reduce the heat to medium and cook until slightly thickened and smooth, about 10 minutes. Gradually stir in the butter until incorporated; add the adobo seasoning and season with salt and pepper. Keep warm.
While the sauce is simmering, make the albondigas: Pulse the carrot in a food processor until finely chopped. Add the reserved roasted poblanos, remaining roasted garlic and onions, chipotle puree, hot sauce, eggs, Cotija and cilantro and pulse until evenly and thoroughly mixed.
In a large mixing bowl, combine the beef, carrot mixture, panko, masa harina, adobo seasoning, 2 teaspoons salt and a few grinds of pepper. Mix well with your hands (do not overwork the meat).
Form the meat mixture into 1 1/2-inch balls (about 40). Arrange on 2 baking sheets and bake until browned and cooked through, 15 to 20 minutes. Toss into the sauce.
To assemble the tacos, heat up the tortillas on a griddle, then spread some chipotle mayonnaise on each. Top with 2 or 3 meatballs and a little of the sauce. Garnish with the pickled onions, queso fresco and cilantro.
Combine 3/4 cup red wine vinegar, 1/2 cup water, 1 tablespoon sugar, 1/2 tablespoon kosher salt, 3 whole cloves, 1 bay leaf, 1/2 teaspoon each dried oregano and pepper and a pinch of red pepper flakes in a medium saucepan. Bring to a boil, then add 1 thinly sliced large red onion (separated into rings). Return to a boil, reduce the heat to medium and cook until the onion softens, 3 to 4 minutes. Remove from the heat, cover and let the mixture cool completely. Refrigerate in a covered container for at least 2 hours and up to 2 weeks.