The night before grilling, combine all of the marinade ingredients in a zip-top bag. Add the flank steak, and seal the bag shut. Toss the steak in the marinade, and refrigerate overnight.
To grill, fire up the grill to medium heat and preheat the oven to 425 F. Clean and peel the sweet potatoes. Cut the sweet potatoes into 1/2" cubes, then toss in 2 tbsp. olive oil, 1 tsp. kosher salt, 1/2 tsp. pepper, onion powder, ancho chili powder, smoked paprika, and brown sugar. Set aside.
In a separate bowl, toss broccoli with 1 tbsp. olive oil, a pinch each of salt and pepper, and the crushed red pepper. Set aside.
Spread the seasoned sweet potatoes across a large, rimmed baking sheet, leaving space between each each cube. Roast sweet potatoes at 425 F for 15 minutes.
While the sweet potatoes are roasting, grill the marinated flank steak for about 3 minutes per side for medium rare with a final temperature of 145 F. Transfer the grilled steak to a plate, and tent with foil to rest at least 3 minutes and keep warm until read to serve.
Toss the spring onions in the leftover marinade, and grill the onions for about 30 seconds per side until char marks appear. Place the grilled onions under the foil with the flank steak until ready to serve.
Remove the sweet potatoes from the oven and toss with broccoli. Roast both vegetables together at 425 F for 10-12 minutes until tender.
Once the veggies are done cooking, remove them from the oven and toss to combine. Divide roasted vegetables between two plates.
Slice the flank steak against the grain and divide between the plates. Chop the grilled onion into bite-size pieces, and use as garnish. Also garnish with freshly slice radish.