To prepare the spinach burgers, start by peeling and finely chopping the onion. Wash the spinach, drain thoroughly, and roughly chop it. Peel the potatoes and grate them raw.
In a skillet, heat two tablespoons of olive oil, add the chopped onion, and let it soften over low heat for about 5 minutes. Then, add the grated potatoes and chopped spinach.
Cook over medium heat for about 5–6 minutes, stirring occasionally and seasoning with salt and pepper to taste. Once the mixture is soft and excess moisture has evaporated, remove from the heat and allow to cool slightly.
Knead with your hands until the mixture becomes soft, compact, and easily moldable. Shape into 4 burger patties (about 160–170 g each) and arrange them on a tray lined with parchment paper. Flatten them slightly with your hands.
Cook the patties in a lightly greased non-stick skillet over medium heat for about 10 minutes, flipping halfway through cooking so they become nicely golden-brown on both sides.