Cut the beef into 4 slices of equal thickness. Heat 2 tablespoons of the oil in a large skillet. Saute the onions for 5 minutes, season with salt and pepper. Lower heat and cook for 30 minutes, until caramel colored, stirring occasionally. Stir in tomatoes and arugula; remove from the heat and reserve.
Add remaining oil to a skillet and heat over medium-high heat until oil is hot. Sear the meat until the outside is well browned, seasoning with salt and pepper as you go. Transfer to a baking sheet.
Place the beef in oven for 5 to 10 minutes, depending on desired doneness.
Melt butter in the same skillet and saute shallots until translucent, about 2 minutes. Season with salt and pepper. Add the brandy and reduce to 2 tablespoons over high heat. Add stock and bring to a boil. Reduce by half and taste for seasoning. Remove from heat.
Mound a serving plate with the onion-arugula mixture. Place the meat on top. Ladle the sauce over the meat and garnish with chopped parsley.