Scrub and pat dry the potatoes, then pierce them with a fork. Place the potatoes on a baking sheet. Season with salt. Bake for about 1 hour, until the potatoes are cooked through (test with a skewer).
Let the potatoes cool, then cut them half (lengthwise). Using a spoon, scoop out the potato flesh, making sure the skin doesn’t tear.
Place the potato skins, skin side down, on an oiled baking sheet. Bake for 8-10 minutes until crispy and golden (oven temperature should be set to around 170-190°C).
Meanwhile finely chop the garlic and heat the oil in a large frying pan. Add the garlic, chilli flakes then the capers and cook for about a minute.
Rinse the seafood and add to the pan together with the lemon zest and juice. Put the lid on and cook for 5 minutes. Add the spinach, and cook for 3-5 minutes, until it’s wilted. Add the scooped out potato flesh and the butter, then stir well until combined and warmed through.
Taste and season with salt & pepper. Add more chilli, if you like it hot. Fold in the Parmesan, if used.
When the potato skins are crispy, carefully stuff them with mixture. Scatter some more grated cheese on top, if liked.
Bake for another 5-10 minutes until golden brown.
Serve warm with the herbs scattered on top.