Sauce can be made up to 3 days ahead of when you want to serve the steak. This will make way more sauce than you need for 2 steaks so I suggest freezing it or planning another use for it.
Preheat oven to 325°F. Line a sheet pan with parchment paper. Place tomatillos, tomatoes, plantain, garlic, and onion on sheet pan cut side down. Roast for 2 hours or until browned and juices begin to caramelize.
While vegetables are roasting, prepare other ingredients.
Place chiles in microwave-safe bowl. Microwave for 1 minute or until soft and pliable. Remove seeds and stems. Reserve seeds.
Heat a dutch oven over medium-high heat. Toast chiles in dutch oven until starting to blister, about 5 minutes. Place in blender.
Toast tortilla until blackened, about 5-10 minutes. Add to blender.
Toast cumin, marjoram, oregano, and reserved chile seeds in the dutch oven until fragrant, about 5 minutes. Add to blender
Toast sesame seeds and peanuts in dutch oven until fragrant, about 5 minutes. Add to blender.
Once vegetables are roasted, add them to the blender as well. Your blender should have the chiles, blackened tortilla, toasted spices and chile seeds, and toasted sesame seeds and peanuts. Add in cherries, honey, salt and 2 cups stock. Blend until smooth.
Heat lard over medium-low heat in dutch oven. Add in blender contents. Cook, stirring frequently, until sauce begins to brown and becomes fragrant, about 5-10 minutes. Add in remaining 3 cups stock, bay leaves, cloves, peppercorns, and cinnamon stick. Stir until well combined. Turn heat down to low, partially, cover the pot, and let simmer for 1-2 hours. Stir occasionally. Watch carefully so it doesn't burn.
Remove cinnamon stick and bay leaves. Discard. Add contents back into the blender. Use a fine mesh strainer to strain the sauce back into the dutch oven. Use a rubber spatula to help push the sauce through the strainer. You should be left with a gritty paste in the strainer and smooth sauce in the pot.
Return dutch oven to stove and heat on medium-low heat. Add in chocolate and 2 tablespoon butter. Simmer 10 minutes or until heated through.
Cool and refrigerate for up to 3 days. Leftovers can also be frozen in quart size freezer zipper bags.
Steak
Rub each steak with 3 tablespoon mole sauce. Let marinate for 2 hours. Let sit at room temperature for the 2nd hour.
Preheat oven to 350°F. Heat a cast iron skillet over medium-high heat. Add in lard and swirl to coat the pan well. Add in steak. Cook 3 minutes per side on all sides. Since an eye of round steak is round, I cook it on 4 sides plus a minute on each end. Place skillet in the oven until internal temperature reaches 120°F. The steak should rest to medium-rare. Remove steak from pan and let rest for 10 minutes. Slice into ½" slices. Sprinkle with flaked sea salt and serve with hot mole sauce on the side.