How to Make It
Place red wine, fennel seeds, crushed red pepper, and thyme sprigs in a small saucepan; bring to a simmer over high. Reduce heat to medium, and simmer, stirring occasionally, until reduced to 3/4 cup, about 10 minutes. Cool completely, about 10 minutes. Pour wine mixture through a fine wire-mesh strainer; discard solids. Place steak in a large ziplock plastic bag. Add wine mixture, seal bag, and turn to coat steak. Marinate in refrigerator, turning occasionally, 4 hours or overnight.
Preheat oven to 400°F.
Place red onion wedges in a 13- x 9-inch glass baking dish. Add vinegar, 1/4 cup water, 1/4 teaspoon salt, and 1/2 teaspoon black pepper; cover with foil. Bake at 400°F for 45 minutes; turn onions over. Uncover and bake until liquid is syrupy and onions are tender, about 20 minutes. Remove and discard root ends from onion wedges; separate the onion wedges into "petals."
Preheat grill to high (450°F to 550°F).
Remove steak from marinade; discard marinade. Sprinkle steak with 1 1/2 teaspoons salt and remaining 1 teaspoon black pepper. Place steak on oiled grill grates, and grill, uncovered, until desired degree of doneness, 5 to 6 minutes per side. Let stand 5 minutes. Slice crosswise against the grain.
Combine cabbage, green onions, oil, red wine vinegar, and remaining 1/4 teaspoon salt in a bowl. Toss to coat. Place cabbage mixture on a platter with steak, onions, and any accumulated juices from the steak.