In a dutch oven or heavy pot, heat vegetable oil over medium-high heat. When it shimmers, add onion wedges. Stir-fry for about 3 minutes, stirring occasionally, until onions start to brown around the edges. Remove onions from pan and set aside.
Add curry paste, ginger, and garlic to the pot. Fry briefly for about a minute.
Reduce heat to medium-low. Add carrots, potatoes, brown sugar, coconut milk, stock, fish sauce, kaffir lime leaves, star anise, and cinnamon stick. Stir to combine.
Cover and simmer for about 15 minutes, or until sweet potatoes are just tender.
Gently stir in onions, cooked turkey, peas, and cashews. Simmer for about 5 minutes longer, or until hot.
Remove and discard kaffir lime leaves, cinnamon stick, and star anise before serving over hot rice. Garnish with cilantro leaves and a squeeze of lime if desired.