In a hot saucepan, combine the butter, oil and onion. Cover and cook over medium-low heat while stirring often, for about 15 minutes. Stir in salt and sugar, raise heat slightly then cook, stirring for another 5 minutes. Stir in garlic and cook for 5 more minutes. Set aside.
In a large frying pan, heat the olive oil over medium-high then sear the steaks, about 4 minutes each side then set aside.
In the frying pan, save juices and stir in stock. Allow stock to reduce by half. Stir in lemon juice, mustard and creme fraiche. Whisk together then bring to a boil. Allow to simmer until sauce begins to thicken, about 1 minute.
Place steaks on a plate, top with onions then spoon on the mustard sauce to serve.