Cut focaccia into 4 equal portions. Split each focaccia piece horizontally into 2 equal halves. Spread cut sides of focaccia with mayonnaise. Set aside.
In an extra-large skillet, heat oil over medium heat. Add onion and cook to caramelize until golden brown, about 20 minutes, stirring occasionally. Remove from skillet and set aside.
Add sliced steak to skillet. Cook and stir for 2 to 3 minutes or until beef is cooked through. Carefully wipe out skillet with a paper towel.
Layer cooked onions, warmed beef, blue cheese and arugula on four pieces of the focaccia. Top with remaining focaccia pieces.
Heat same skillet over medium heat. Place 2 sandwiches in skillet and cook until cheese is melted and sandwich is warmed through, about 3 minutes per side. Repeat with remaining 2 sandwiches.
Tip: To make Panera clean recipes at home, follow these recipes, use the ingredients listed and for more information about Panera clean, go to www.panerabread.com/cleanfood for our No No list.