If you don't already have a burger stuffer I recommend you get one,they are great to have! I bought mine at a HomeGoods Store for $6.00,best thing I ever bought there! I you don't have one a top from a gallon jar will do otherwise you'll have to make do with your hands!
Start by dividing the ground chicken into four equal 1/4 pound balls of meat.Season with a pinch of sea salt and black pepper.Spray the burger stuffer with some cooking spray to prevent sticking. Divide each ball into half and insert one half into the press,using a teaspoon create a cup out of the meat.
Fill each cup with one two tablespoons each of wilted spinach,sauteed mushrooms,feta cheese and one tablespoon of pitted kalamata olives. Cap with the other half of ground chicken and press down along the edges with the back of your teaspoon.(Dip the spoon into some cold water to prevent it from sticking to the meat.)
Pan sear each patty in the olive oil for 4-6 minutes per side,melting some extra feta cheese on the tops just before finishing cooking. Serve on toasted kaiser rolls with lettuce,tomato,sliced red onion and a tablespoon or more of Tzatziki sauce.