Preheat the oven to 220ºC (430ºF). Coat cubed butternut squash with olive oil, sprinkle with salt, and spread on a baking sheet. Roast for 25-30 minutes until soft and caramelized.

Heat 2 tablespoons of olive oil in a large pan over low-medium heat, add diced garlic, and sauté for 1-2 minutes until fragrant. Stir in chili flakes.

Cook spaghetti in salted boiling water according to package instructions, but reduce the cooking time by one minute for al dente texture. Reserve a cup of pasta water before draining.

In the pan with garlic, mash half of the roasted butternut squash with a fork. Retain the other half for texture.

Add drained spaghetti and washed spinach to the pan. Stir well to coat the pasta. Add lemon juice, salt, and pepper to taste.

Plate the spaghetti and top with chopped walnuts and lemon zest. Serve warm.