Combine garlic, shallots, ginger, serrano chiles, lime juice, cumin, turmeric, and coriander in a blender or food processor, and pulse until ingredients are finely chopped. Add coconut oil and pulse until ingredients form a smooth paste.
Sprinkle chicken thighs with 1 ½ teaspoons kosher salt and black pepper. Place chicken, skin side down, in a large high-sided skillet over medium. Cook until fat is rendered and skin is crispy, about 12 minutes. Flip thighs, and cook until a thermometer inserted into thickest portion of meat registers 165°F, about 10 minutes more. Transfer to a cutting board.
Reserve 1 tablespoon drippings in a small bowl. Wipe skillet clean and add reserved drippings back to skillet. Heat over medium-high and add ½ cup ginger-turmeric curry paste and fish sauce. Reduce heat to low and cook, stirring constantly, until aromatic, about 2 minutes.
Stir in coconut milk and water. Simmer until flavors meld, about 4 minutes. Season broth with remaining 1 teaspoon salt. Turn off heat and stir in spinach; cover with a lid until wilted, about 1 minute.
Slice chicken thighs into 1-inch pieces. Add about ½ cup of cooked rice to 4 shallow bowls. Ladle broth around rice. Top each with sliced chicken. Serve with scallions, cilantro, and chiles, if desired.
