1. Preheat oven to 425°F. Preheat large cast-iron skillet on medium 4-5 minutes. Line baking sheet with wire rack.
2. Prepare picanha. Combine in spice grinder: bay leaf, fennel seeds, cumin seeds, coriander seeds, dried thyme, dried oregano, salt, and pepper. Pulse and shake spice grinder for 30–35 seconds until ground and fine powder. Score fat cap on roast, ½-inch-deep, in crisscross pattern. Coat roast evenly with spice rub, then arrange roast, fat side down in cast-iron skillet (wash hands). Cook 7–8 minutes, then flip over and continue cooking 5 more minutes.
3. Transfer roast to prepared baking sheet; bake 25–30 minutes, until 125°F (for medium rare; warm red center), 130°F (medium; warm pink center), or up to 170°F (well done)*. Transfer roast to cutting board and let stand 10–15 minutes before slicing; temperature will rise 5°F–10°F during this time.
4. Meanwhile, prepare sauce. Heat oil in medium sauté pan on medium-high 3–4 minutes. Add onions, and garlic paste; cook 2–3 minutes, stirring occasionally, until onions are tender. Add green chilies, tomatillo salsa, and wine. Cook 4–5 minutes, until liquid is reduced by one-half. Remove pan from heat and set aside to cool.
5. Add to blender: cooled onion mixture, Worcestershire, cilantro, and parsley. Blend on high speed 1–2 minutes until sauce is smooth and well blended. Thinly slice roast against the grain. Serve with green garlic sauce.

*NOTE: The doneness temperatures for medium rare and medium stated above are traditional temperatures used for cooking beef but are not recommended by the USDA. To maintain food safety, the USDA recommends an internal temperature of 145°F, as measured using a food thermometer, with a 3-minute rest time before carving.