Mix all marinade ingredients in a big saucepan.
Heat on low until it bubbles, stirring occasionally.
Simmer for 5 minutes, then cool.
Slice steak into 2-3 inch pieces.
Put steak in a Ziplock bag with marinade, and swish.
Marinate 8-24 hours.
Heat sauté pan, melt butter, add onions, sugar, salt, and pepper.
Cook for 30-40 mins, stir.
Drain marinade from steak.
Grease grill pan, cook steak 2-3 mins each side.
Mix caramelized onions with sirloin, and serve with sides.