Heat oil in a large pot over medium heat. Add onion and sauté 2–3 minutes until softened.

Stir in garlic and ginger; cook 30 seconds until fragrant.

Add red curry paste and cook 1 minute, stirring to coat onion mixture.

Add sliced chicken and cook 3–4 minutes until lightly browned on the edges.

Pour in chicken broth and coconut milk (if using), scraping up any browned bits.

Stir in fish sauce, lime juice, and brown sugar. Bring to a gentle simmer.

Add mushrooms and simmer 5 minutes until chicken is cooked through and mushrooms are tender.

Stir in spinach or bok choy and cilantro; cook just until greens are wilted. Taste and adjust seasoning, adding more lime juice, fish sauce, or sugar as needed.

Serve hot, garnished with sliced chili and lime wedges if desired.