How to Make It
Heat 1 tablespoon of the oil in a medium skillet over medium-high. Add onion and cook, stirring occasionally, until it begins to soften, about 3 minutes. Reduce heat to medium-low and cook, stirring often, until onion is very soft and deep golden brown, 18 to 20 minutes.
Meanwhile, stir together goat cheese, mayonnaise, chives, and pepper in a small bowl. Divide goat cheese mixture among cut sides of hoagie rolls, spreading evenly to cover. Divide roast beef, arugula, and caramelized onions among bottom halves, then sandwich with tops, goat cheese side down. Brush outsides all over with 1 tablespoon of the oil.
Heat a large nonstick skillet over medium. Place sandwiches in skillet and place a second heavy skillet on top. Cook, flipping once, until bread is golden brown and crisp, about 3 minutes per side.
Toss tomatoes, salt, and remaining 1 tablespoon oil in a medium bowl. Serve with sandwiches.