Combine water and yeast in a mixing bowl. Stir and let it dissolve and bubble for few minutes.

Add in rest of ingredients and mix (with electric mixer or by hand) until the dough is smooth. 

Cover and let rise for at least an hour.

Put all ingredients for pistachio filling in a food processor and process until as smooth as possible (I like it a bit chunky). Transfer filling to a little bowl.

Cover filling with a thin layer of olive oil and refrigerate.

After the dough has risen, divide it into 4 pieces. Each challah will be braided from 2 strands. 

Roll each part with a rolling pin to 20 x 40 centimeters (7 ¾ x 15 ¾ inches) aproximately ½ centimeter (¼ inch) thick. Spread a ¼ of filling on the dough and roll. When you have 2 rolled strands, braid them togther, and roll it around itself. Repeat with the other 2 pieces of dough.

Place rolls on a baking sheet covered with parchment paper.

Cover challahs with a clean and damp kitchen towel and let rise for another hour.

Preheat the oven to 180°C (350°F). Brush the challah with beaten egg twice and top with crushed pistachios.

Bake for 25 to 30 minutes untill golden. Cool on a wire rack.