Toss butternut squash cubes with lemon ginger seasoning and salt. Heat 1/2 teaspoon of oil in a large fry pan and sauté squash for about 6 minutes or until lightly browned. Transfer to a plate.
Heat 1/2 teaspoon of oil in the same pan over medium high heat and cook red onion and garlic for 5 minutes until soft.
Scoop the cream at the top of the can of coconut milk (about 1/4 cup) and add it to the pan. Bring to a boil over medium heat. Stir in curry paste and turn heat down to low. Simmer, without stirring, until fragrant and coconut cream starts to release oil, about 3-5 minutes. Add the rest of the coconut milk and water and bring to a boil. Reduce heat and add squash. Cook until tender, about 8 minutes. Stir in spinach, fish sauce (taste curry and add more fish sauce if desired), and sugar.
Remove from heat and allow to sit for at least an hour before serving, as the curry becomes thicker and more flavorful with time. Serve with jasmine rice.