If you're making your own yellow curry paste, do that first. Slice your squash in half and scoop out seeds. Carefully peel the rind off, and then chop into 1/2" pieces. You'll need about 5-6 cups of squash.
Finely slice your leek and wash. Melt the oil over medium heat and add leeks. Soften for 4-5 minutes, and add about 1/2 tsp of salt.
Stir in the curry paste to completely cover the leeks and stir for a few minutes to allow the spices to bloom. Add in the squash pieces, stir to coat, and cover and cook for 6-7 minutes, stirring occasionally so they don't burn.
Add in the stock and the coconut milk. Bring to a boil, then reduce heat to a simmer for 30-35 minutes or until squash is extremely soft.
Use an immersion blender or stand-up blender like a Vitamix and process till very smooth. Add the lemon juice, then taste to adjust seasoning. If it's too spicy, add some sugar (like honey or maple syrup), too salty add more acid (lemon juice), or add some salt if needed.
Serve with a splash of coconut milk.