Heat grill to medium-high; heat oven to 400°.
Thread tomatoes onto skewers and spritz with nonstick cooking spray. Grill corn 12 minutes, turning frequently, and grill tomatoes 8 minutes, turning once. Transfer to a cutting board and remove tomatoes from skewers into a bowl. Cut kernels from corn cobs and add to bowl along with lime juice, oil, salt and, if using, cilantro. Set aside.
Peel plantains and cut into 1/2- to 3/4-inch slices. Toss in a bowl with 1 1/2 tbsp of the oil and 1/4 tsp of the salt. Place on a nonstick or foil-lined baking sheet and bake at 400° for 15 minutes.
Meanwhile, carefully split flank steak in half horizontally as if opening a book, cutting into 2 thin pieces. Slice each piece in half (for a total of 4 pieces). Season with ancho chile powder, salt and pepper. Grill steak to taste, about 5 minutes, turning once, for medium-rare. Transfer to a platter to rest. Add scallions to grill; grill 6 minutes, turning once, until charred.
Remove plantains from oven, smash with the bottom of a glass to flatten, and flip over. Brush with remaining 11/2 tbsp oil, sprinkle with remaining 1/4 tsp salt and bake at 400° for an additional 5 minutes. Serve steak and scallions with salsa and tostones alongside.