Mince garlic, slice red onion.
Combine olive oil, balsamic vinegar, and garlic in a small bowl.
Brush one side of each slice of bread.
Place bread, oil side facing out, onto a hot Panini pan.
Stack turkey, cheese, onion, and spinach on bread.
Place a second slice of bread on top of the filling.
Cook until bread is grilled and cheese is melted.