Preheat a panini press according to manufacturer’s instructions.

Place the bacon in a large, deep skillet, and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain the bacon slices on a paper towel-lined plate. Meanwhile, melt the butter in a large skillet over medium heat. Cook and stir the garlic and onion until the onion has softened and turned translucent, about 10 minutes. Stir in the spinach and cook until wilted, about 3 minutes more.

While the onions and spinach are cooking, stir together the chilli mayonnaise. Spread the mayonnaise over 4 slices of focaccia bread. Place a slice of cheese onto the 4 slices, then divide the turkey among the sandwiches. Place 2 strips of bacon onto each sandwich and top with the spinach mixture. Place the remaining slices of bread onto the sandwiches.

Cook the sandwiches in the preheated panini grill according to manufacturer’s directions until crispy and golden brown, about 5 minutes.