Hull the strawberries then cut them into small chunks and then add them to a pan and heat on high for 3 minutes.
Using a potato masher or a fork, mash the strawberries until they turn a jam (jelly) consistency.
Cook on low for 5 minutes and then transfer to a bowl to cool. It will have some liquid in the bowl, but it will set after it has been left to cool. Strain through a fine mesh sieve if you don't want the seeds.
Notes
You can use frozen strawberries, but make sure they are fully defrosted first.
You could blend other fruits in to this like peach or raspberries.
You can add honey (not for babies) or maple syrup if you want it sweeter.
If you don't like the strawberry seeds, then pour it through a fine mesh sieve.
You can freeze this puree in an ice cube tray and the store the cubes in a freezer bag.