Start by adding flour, butter, yogurt (start with 3/4 dl) and salt together. Kneed until you have a firm dough. This will take a while (4-5 min). If it really doesnt stick together add a little more yogurt and kneed again.
Wrap in cling film or put in a plastic bag and let it rest in the fridge for 20-30 minutes. (This step can be skipped).
When the dough is ready preheat the oven to 190 degrees Celsius.
Roll the dough either in to one big think circle, or two smaller ones (if making two galettes, I prefer making small ones).
Add the ricotta on top of the crust spreading it evenly, leaving 1/2-1 cm around the edges. Add tomatoes on top and season with salt and pepper. Fold the dough edges in over the the ricotta and tomatoes.
Brush with egg wash and bake in the oven for 20 minutes, or until golden brown.
Garnish with fresh rosemary.