Prepare the Dijon onions: Heat 1 tablespoon olive oil in a large sautee pan over medium-high. Add the sliced red onions and 1/4 teaspoon kosher salt. Cook until the onions begin to soften and brown, 5-8 minutes. Remove from heat and stir in the Dijon to evenly coat. Remove the onions to a plate or bowl, then gently wipe the skillet clean with a paper towel.
Heat the remaining 1 1/2 tablespoons olive oil in the skillet over medium-high Sear the steaks for 1 minute on each side, then reduce the heat to low and cook the steak for about 6-8 minutes, turning once, until rare and a thermometer inserted in the center reaches 125, then slice into strips.
Place buns into stoneware pizza baking pan. Lay the bottom of the buns in a single layer in the baking. Arrange steak slices and brie, place in oven to broil for 1 minute.
When removed Top with caramelized onions. Spread the inside of the top buns with about 2 teaspoons jam and arugula then lay the buns on top to complete the sandwiches.