In a slow cooker, add the coconut milk, curry paste, fish sauce, and sugar, stir to combine. Add the butternut squash and chicken, pushing to submerge in the liquid. Cook on low for 8 hours or high for 4 hours.
In the last 1 hour of cooking on low (30 minutes if on high setting) add the mushrooms. If cooking on low, switch the cooker to high and transfer the chicken to a bowl. Use two forks to shred the chicken (it will shred very easily) and return to the cooker. To thicken liquid, stir in the cornstarch-water mixture. Then, add the spinach and cook for 5 to 10 minutes, until the leaves are wilted.
Cook the curry for 5 to 10 minutes, until the spinach is wilted. Add the lime juice and season with salt and pepper.
Serve over rice and top with lime, scallions, cilantro, mint, and Sriracha, as desired.