-
Cut the sirloin into bite-sized pieces. Season with salt and pepper.
-
Peel and dice the onion.
-
Juice and zest the lemon.
-
Heat the olive oil in a skillet.
-
Add the beef and sear on all sides.
-
Season with salt and pepper.
-
Cook until desired pinkness.
-
Transfer the beef to a serving plate and cover to keep warm.
-
Add the onion. Sauté until transparent.
-
Add the beef stock and bring to a boil.
-
Whisk in the lemon juice, zest, and mustard.
-
Transfer a few tablespoons to a small dish and whisk in the cornstarch.
-
Slowly whisk the mixture back into the skillet.
-
Keep whisking until the sauce thickens.
-
Season with salt, pepper, and sugar.
-
Pour the sauce over the cooked beef.
-
Garnish with parsley and serve.