Coat cubed butternut squash in 1 tbsp of olive oil and season with salt. Bake in a hot oven (225º C / 435º F) for about 40-50 minutes, until butternut squash turns soft and gets lightly caramelised.
Heat up 2 tbsp of olive oil in a pan. Fry off garlic gently on a low-medium heat, stirring frequently so it doesn’t burn. Add chilli flakes and cook for another minute or two stirring frequently.
Cook spaghetti for a minute shorter than you would normally.
Mash half the butternut squash with a fork or potato masher (you can also use a food processor or blender for a smoother texture). Put both mashed and whole butternut squash chunks into the pan with chilli and garlic.
Add drained pasta and washed spinach into the pan. Incorporate them well into the pumpkin. Taste and season with pepper, a squeeze of lemon juice and additional salt and chilli if needed.
Serve immediately with chopped walnuts and lemon zest on top.