Roasted Red Onions: Toss all ingredients in a half hotel pan; roast at 400°F until caramelized and very tender, about 35 minutes. Refrigerate in an airtight container up to 6 days. Makes about 2 cups.
For serving, place bottom half of 1 baguette section on a quarter sheet tray; top with 2 ounces cheese. Broil just until cheese melts, about 2 minutes; top with 2 ounces sandwich prosciutto, 1/2 cup arugula, 1/2 cup Roasted Red Onions and top half of baguette section.