Season beef with salt and pepper. Using a sharp knife, score the fat rim. Heat oil in a large deep pan over medium-high heat. Fry beef 2 minutes on each side. Remove and set aside. Pour off excess oil.
Peel onions, halve them, and cut into thin half-rings. Add 1 tablespoon butter to the pan. Reduce heat to medium.
Fry onions for 3-5 minutes until softened. Dust roughly with powdered sugar. Add tomato paste and cook for 1-2 minutes. Pour in some wine and let it boil down; repeat until all wine is used (about 10-15 minutes total). Add beef broth.
Return beef to pan. Cover and simmer over medium-low heat for about 1 hour, spooning sauce over meat every 10-15 minutes.
Before serving, season sauce with salt and pepper. Stir in the remaining 1 tablespoon cold butter.