Simple syrup is easy to make, just bring to boil water and sugar and stir till sugar is dissolved completely. Remove from heat and let cool. (You can make up the simple sugar ahead of time and have in the refrigerator).
Squeeze lemons into a bowl, trying to keep the seeds out. Place juice from lemon’s into the cooled simple syrup and mix well.
Blend raspberries in a Ninja or blender and add to lemonade mixture. Then add to cold water and stir. Pour over a glass of ice. Add a thin slice of lemon to the glass and serve.