The longer you chill the sirloin steak after rubbing it with the curry powder and ginger-onion mixture, the more flavorful it will be. Curry-spiced sweet potatoes roast in the oven with the steak.
Combine onion, garlic and ginger in a food processor and process until like a paste. Put in a fine strainer and drain as much water as possible.
Trim beef of excess fat; sprinkle with the salt. Combine 1 teaspoon of the curry powder with the onion mixture and rub this over the top of the beef. Poke a few holes in beef and press some of the mixture into each. Wrap tightly in plastic wrap and refrigerate for several hours or overnight.
To cook, remove beef to countertop for at least 20 minutes. Heat oven to 450 degrees F.; toss sweet potato chunks with oil and then sprinkle with remaining 2 teaspoons curry powder. Coat a nonstick baking pan with nonstick spray; add potatoes and roast at 450 degrees F. for 10 minutes. Place beef in a small nonstick roasting pan; add to oven and roast 15 minutes. If top begins to burn, loosely cover with foil. Roast another 5 to 10 minutes or until potatoes are tender and internal temperature of steak reaches 135 degrees F. on an instant-read thermometer.
Remove beef and potatoes from oven, tent beef with foil and let rest 10 minutes. Slice sirloin and serve with sweet potatoes.