Start by making the caramelized onions. Add the butter to a pan over medium heat. Add the chopped onion and cook for 3-4 minutes. Once onion has begun to cook down and stick to the pan, add a splash of water. When the water has evaporated, add another splash, stirring throughout. Continue this process until the onions have caramelized and look darker in color. It should take around 15 minutes. Transfer the onions to a bowl.
To make the cream sauce, add the butter, shallots, and garlic to a pan over medium-low heat. When the garlic and shallots have cooked down, add the broth, dijon mustard, and brown sugar and cook for one minute. Add the heavy cream, salt, and pepper. Cook for an additional minute until the mixture starts to gently boil. Take off the heat.
Heat a grill or pan to high heat. Rub the steaks with olive oil, salt, and pepper. Add the steaks to the hot grill / pan and sear on both sides. After it is seared, cook on both sides, flipping occasionally, until the steak is done to your preference. Remove and serve immediately.
To serve, top the steaks with the cream sauce, caramelized onions, and arugula. Enjoy!