Wash the strawberries in cold water and drain the excess water.
Put the strawberries into the saucepan along with the sugar and heat on a medium heat. If the strawberries are fresh, add a small splash of water to the pan too (not required for frozen strawberries).
Heat, stirring occasionally until the strawberries start to soften and break down. Mash them a little with a potato masher or wooden spoon to help them.
Let the strawberries come to a boil, keep stirring every now and again and boil them for about 15 minutes or until the mixture has thickened slightly and looks a little gelatinous.
Remove from the heat and let cool for 15 minutes.
Put the strawberry reduction into a food processor or blender and blitz for a few seconds until completely smooth.
Pour the strawberry puree into a bowl through a fine mesh sieve to remove any excess fiber or seeds.
Let cool completely before using.
Notes
It's best to use a high-sided saucepan as the strawberry puree can often spit as it cooks and make a mess on your countertop, or even splash your hands - it's very hot! Straining out the seeds and excess fiber is optional, you can skip this step if you like.