Preheat the oven to 425°F (220°C) and grease a baking dish with butter. Set aside.
Place the zucchini slices in a strainer and season with salt. Set aside while you prepare the rest of the ingredients to help extract any unwanted liquids.
Roughly slice the onions and heat the butter in a saucepan. Sauté the onions for 5 minutes, then mince and add the garlic. Season with salt and pepper. Cook for another minute until fragrant, and pour in the sour cream. Simmer for 2-3 minutes. Turn the heat off and add the feta and mozzarella cheese. Stir to melt into a sauce.
Pat the zucchini slices dry with a piece of absorbent paper.
Lay the zucchini slices into the prepared baking dish. Pour in the sauce and spread evenly.
Bake in the preheated oven for 15 to 20 minutes until golden brown. Serve hot, straight from the oven.