Start by lightly steaming the kale. I used a steaming basket which sat in a saucepan to do this in two batches. You want to get the kale a little softened, without properly cooking it. It will take just a couple of minutes and will turn bright green.
Now start cooking your pasta. Bring a large pan of water to the boil and add your spaghetti. While it's cooking, you will make the pesto.
If you have a large food processor, put the kale in with the walnuts, avocado, garlic, lemon juice and olive oil and pulse until it's blended but still chunky.
If you're using the hand blender method as I did, put the kale into a large bowl. Pour in the lemon juice. Place the walnuts and garlic on top of the kale and pour the olive oil over them. Press the hand blender down over all of the garlic cloves and blend until the garlic is completely pureed in that spot. Now move the blending stick around and blend all of the walnuts (these can be a little chunkier) and kale to achieve the desired texture. Some bits of kale could be left almost whole if you'd like it more textured.
When the pasta is ready, drain and return the pan.
Top with the pesto, add salt and pepper to taste, and then use tongs to distribute it. It can take a lot of work to get the pesto mixed in, because the kale sticks together a lot! You may need to add a little more oil.