1. Add the strawberries, juice of both lemons and sugar to a high powered blender and blend on high until smooth and completely pureed. Pour the mixture into a saucepan placed over medium heat, and add the egg yolks and lemon zest. Whisk together until combined.
2. Continuing whisking until the mixture just begins to bubble, then turn the heat down to medium-low and cook for about 15 minutes, whisking continuously, until the mixture begins to thicken up. When you stir it with a spoon, the curd should thickly coat the back of a spoon.
3. Remove the curd from heat and whisk in the butter cubes until melted. Pour the curd into a heat-safe container and allow it to completely cool at room temperature, before placing it in the fridge to fully cool down and thicken up.

Curd can be kept in an airtight container, in the fridge, for 3-4 weeks.