3 tablespoons olive oil, divided
1 large yellow onion, thinly sliced
4 ounces goat cheese, softened
6 tablespoons mayonnaise
2 tablespoons chopped chives
¼ teaspoon black pepper
4 8-in. hoagie rolls, halved lengthwise
12 ounces thinly sliced deli roast beef
1 cup packed baby arugula
4 medium tomatoes, cut into 8 wedges each
½ teaspoon kosher salt