Preheat oven to 425˚F. Line a baking sheet with foil.
Place squash on baking sheet. Toss with 1 tbsp oil and ¼ tsp salt, then spread in an even layer. Bake 15 minutes, flip, and continue baking until squash is tender and has some browning on the outside.
Meanwhile, crush cumin seeds in a mortar and pestle, OR place them on a cutting board and crush by pressing down with the bottom of a saucepan.
Heat remaining 2 tbsp oil in a medium saucepan over medium-low. Add cumin seeds and mustard seeds; toast until sizzling and fragrant, stirring constantly, about 1 minute.
Add onion, garlic and remaining ½ tsp salt; cook, stirring frequently, until softened, about 6 minutes. Stir in turmeric and cook until fragrant, about 1 minute.
Stir in lentils and water. Increase heat to medium-high and bring to a simmer. Cover, reduce to medium-low, and cook until lentils are tender, about 10 minutes, stirring once or twice. Stir in butternut squash, then stir in spinach just until wilted, about 30 seconds. Serve in bowls, topped with green onions.