Tuna Cakes Sandwiches with Parmesan Toasted Sourdough

recipe tuna cakes (recipe here: https://lifecurrentsblog.com/tuna-cakes/)

about 1/3 – ½ cup freshly grated Parmesan cheese

salted butter at room temperature or spreadable butter (enough for a light smear)

slices sourdough bread

Fresh spinach

Tomato slices

Thinly sliced red onion

Toppings of your choice