In a saucepan over medium-low heat put the oil and garlic. Leave it for about two minutes, and add the broccoli and the water or broth. Cover and let cook for 15-20 minutes.
When the broccoli begins to change color and soften add the fresh basil if using and the cashews. Cover and let it simmer for about 3-5 more minutes.
Blend and return to the pot.
Test for seasoning and adjust if necessary; also check if it needs more liquid. If you need more liquid, add vegetable broth or water.
To make the topping, chop the basil and put it in a bowl with the chopped cashews and the lemon zest. Mix well, add the salt and gradually add the olive oil.
Test and check if you need a few drops of lemon juice.