This is my favorite sandwich at Panera so I was happy to find the recipe and share! It's a terrific way to use up leftover spinach artichoke dip, or an excuse to make some up...try Recipe#277769 or Recipe#243807 to cut out some of the fat.
Preheat panini grill to medium temperature. In a medium sauce pan, sauté diced red onion in olive oil until transparent. Add balsamic vinegar and salt and pepper to taste.
Slice the loaf of pesto focaccia into two semi-circles, and halve each semi-circle. Layer the turkey onto the bottom halves of the focaccia bread, and spread spinach artichoke mix over turkey. Layer sautéed onions, asiago/parmesan blend and tomato slices. Close face of panini with the top halves of focaccia bread.
Add panini to preheated grill and press lightly. Grill 5 minutes on medium or until the cheese begins to melt and there are grill marks on the focaccia bread.