Add spaghetti to a saucepan of boiling water and add a pinch of salt. Leave to cook for 10-15 minutes on a low/medium heat and reserve 1/4-1/2 cup of the spaghetti water.
Meanwhile, add beetroot, garlic, walnuts, lemon juice, salt, pepper, 1/4 cup of spaghetti water to a food processor and blitz for a few minutes. Gradually add olive oil until the mixture becomes smooth.
Add the spaghetti to a large bowl. Add the beetroot pesto to the bowl, mix well and adjust seasoning if necessary. Garnish with parsley.