How to Make It
Stir together parsley, marjoram, oil, vinegar, garlic, red pepper, colatura, hot paprika, sweet paprika, and 1 teaspoon salt until combined. Set chimichurri aside. (Chimichurri may be stored in an airtight container in refrigerator up to 3 days. Bring to room temperature before serving.)
Open bottom and top vents of a charcoal grill completely. Light 2 charcoal chimney starters filled with briquettes (about 8 1/2 cups of briquettes per chimney). When briquettes are covered with gray ash, pour in an even layer on bottom grate of grill. (Internal temperature should be about 375°F to 400°F.)
Place steak on a rimmed baking sheet, and season with black pepper and remaining 4 teaspoons salt. Place steak on oiled grate, fatty side down, and grill, uncovered, until charred and browned, about 15 minutes. Flip and grill, uncovered, until a thermometer inserted in thickest portion of steak registers 135°F for medium-rare, about 13 minutes, or to desired degree of doneness. Remove steak from grill, and transfer to a cutting board; let rest 10 minutes before slicing.
While steak rests, cook onions. Carefully remove cooking grate from grill and set aside. Push coals to one side of grill, and arrange onions in a single layer in cleared space; scatter coals over onions. Cook until onions are charred and insides are crisp-tender, about 8 minutes. Transfer onions to a metal baking dish, and let stand until cool enough to handle, about 5 minutes. Brush away ashes with a paper towel.
Cut steak against the grain into 1/2-inch slices, and cut onions in half lengthwise. Serve with chimichurri.