In a medium frying pan on medium heat, add the olive oil, onion, and garlic and cook until the onions are translucent.
Add the fresh dill and spinach and cook until wilted. Set aside.
Heat a large frying pan on medium heat and butter 4 slices of bread and put them in the frying pan (buttered side down).
Add one provolone cheese slice to each slice of bread and let them cook.
While the bread is browning, put the spinach mixture back on medium heat and add the feta cheese. Stir until the feta starts to get a creamy consistency.
Add a tablespoon of the spinach/feta mixture to each slice of bread, then sandwich two together.