Allow the frozen spinach to thaw completely then squeeze out any excess moisture.
Make the white sauce according to recipe instructionsthen stir the spinach through the sauce.
Add the grated cheese and transfer the spinach to a casserole dish or skillet.
Melt the butter in a large pan over medium-high heat then ad the pan and minced garlic clove.
Toast for 2-3 minutes, stirring regularly, until the breadcrumbs are a light golden brown color.
Season with salt and black pepper then remove rom the heat.
Sprinkle the panko over the spinach then top with grated parmesan cheese.
Place the spinach gratin in a preheated oven for 15-20 minutes until hot and bubbling then remove from the oven. Allow to cool for 5 minutes then serve.