In a saucepan (big enough to hold the broccoli) bring the water to a boil.
Wash your broccoli head
Roughly cut your broccoli into florets, remove the coarse skin from the stalk, and slice the stalk (it's nutritious and delicious and shouldn't be wasted.)
Add the broccoli to the boiling water, cover and simmer the broccoli for 8 minutes or until the broccoli is bright green and tender. You should be able to squash the broccoli with a fork.
Drain the broccoli, reserving any liquid.
Blend or process until a smooth texture is reached, add liquid as needed. If you are blending the broccoli while still hot, be sure your blender/processor can be vented. Hot foods should not be blended in a sealed blender (like a bullet-style blender) as the pressure can build and cause an explosion.
Notes
Choose fresh broccoli: The head of the broccoli should feel tight and dense, there should be no yellowing of the florets and the stalk should feel firm.
You can reinvigorate limp broccoli that is a little past its best and, remove a slice from the stem, sit the stalk in a small bowl or glass of water, and stand in the water in the fridge. The broccoli should firm up again.
When broccoli is out of season you can absolutely use frozen broccoli. Frozen broccoli is typically snap-frozen quickly after harvest so it retains its nutrition
If you do not have a blender, a food processor or handheld stick blender will work as well.
Check your broccoli or set a timer while it is simmering, as once you've boiled it for too long it will lose its vibrant colour and you won't be able to get it back!
For the most vibrant green colour, once you have drained your cooked broccoli, dump it into a bowl of iced water. (I didn't include this as a step as I don't do this every time, but it does make for an even better-coloured puree.)
For babies do not add salt
For adults and older children, you can turn broccoli puree into a delicious side dish for meat fish and chicken meals. Add salt, olive oil or butter and a little lemon juice.
Freeze leftover broccoli puree in ice cube trays so that you can use it easily in future meals and recipes.
Store your broccoli puree in a clean airtight jar or container in the refrigerator for up to 4 days, or freeze for up to four months.