Sprinkle squash with 1/2 tsp. salt. Heat 1 tbsp. oil in a large nonstick frying pan over medium-high heat. Brown half the squash in oil, turning once, 6 to 8 minutes; reduce heat if squash starts getting dark. Transfer to a bowl and repeat with 1 tbsp. oil and remaining squash.
Meanwhile, heat remaining 1 tbsp. oil in another large frying pan over medium heat. Cook onion, stirring occasionally, until deep golden, 12 to 15 minutes. Transfer half to the nonstick frying pan and reserve other half in a bowl.
Add chiles, cinnamon, and curry leaves to onion in pan. Cook, stirring often, until curry leaves are very fragrant, about 2 minutes. Add turmeric, cumin, and remaining 1/2 tsp. salt and cook, stirring, until spices are fragrant, about 1 minute.
Return squash to nonstick frying pan (with onion) and add coconut milk. Bring to a boil over high heat, then cover, reduce heat, and simmer until squash is tender, 5 to 10 minutes. Stir in cashews and lemon juice, and add more salt to taste. Top curry with reserved onion and serve with rice.
*Kabocha peels are edible. Find curry leaves in Indian markets.
Note: Nutritional analysis is per serving without rice.