Whisk together the marinade: coconut milk, sesame oil, garlic, grated ginger, brown sugar, chopped cilantro, and sweet chili sauce in a large bowl.
Season the flank steak with 1 teaspoon salt and 1 teaspoon pepper, place it in a large baking dish or a zip-top bag, pour the marinade over the steak, and refrigerate for 2 to 24 hours.
Preheat the oven to 400°F (200°C). On a baking sheet, toss the cubed mango with 1/4 teaspoon chili powder and 1 teaspoon olive oil.
Roast the mango on the baking sheet for about 20 minutes, until slightly golden and caramelized; remove and let cool slightly.
In a food processor or blender, combine the roasted mango, juice of 2 limes, 1 tablespoon white vinegar, a pinch of salt and pepper, and process while streaming in 1/2 cup olive oil until the vinaigrette is smooth and emulsified.
Cook the marinated steak: grill or broil to your preferred doneness (for a steak around 1 inch thick, broil about 5 minutes per side for medium to medium-well). Let the steak rest 5–10 minutes, then slice thinly against the grain.
In a large bowl, toss the spring greens with the chopped mango for the salad, diced sweet onion, 2 tablespoons chopped cilantro, and season with a pinch of salt and pepper.
Divide the dressed greens among plates, top with sliced steak and sprinkle with cashews. Drizzle the roasted chili mango vinaigrette over each salad and serve.