Preheat the oven to 180 °C and grease the holes of a muffin pan (8 large or 12 regular). Heat the butter and fry the onion and garlic over low heat for 5 to 6 minutes, or until the onion has softened. Transfer to a mixing bowl to cool.
Ensure that the cooked spinach is well drained, chop it finely and add it to the onion mixture. Add the feta, mozzarella and cheddar cheese and sour cream. Season well with black pepper and mix together.
Place the phyllo sheets on a clean work surface. Cut each phyllo sheet into 8 to 12 squares. Cover with a clean, slightly damp dish cloth to prevent the phyllo from drying out.
Brush a phyllo square with melted butter and place it in the base of one prepared pan, allowing the corners to extend over the top of the pan. Repeat with four or five more phyllo pieces, rotating them slightly to completely cover the side of the pan. Repeat to line the remaining prepared pans.
Divide the spinach mixture among the phyllo cases and fold the pastry over the filling to cover. Brush with a little extra butter and scatter sesame seeds on top.