Cut focaccia in half through the side to make two rounds (top and bottom halves of sandwich).
Mix mustard, scallion, garlic, tarragon and black pepper in small bowl. Spread over top and bottom halves of sandwich.
Arrange arugula or spinach over mustard on both halves. Arrange sliced Brie, then sliced ham, over the bottom half. Top with the other half and press down gently.
Coat a panini press or nonstick skillet with cooking spray and heat over medium heat. Add panini and cook until browned and cheese softens, but doesn’t completely melt, 4 to 5 minutes per side. If using a skillet, press a heavy weight, such as another skillet, on top of sandwich until bottom is browned, then flip and repeat on the other side.
Remove to a cutting board and slice like a pizza into six wedges.
Tip: The white surface on Brie cheese is perfectly edible and softens nicely in this sandwich.