Heat olive oil in a large pot over medium heat.
Add the chopped onion and cook until soft and translucent, about 3 to 4 minutes.
Stir in garlic and ginger, cooking for about 30 seconds until fragrant.
Add the red curry paste and stir well to coat the aromatics.
Add the sliced chicken and cook until lightly browned on the outside.
Pour in the chicken broth and coconut milk, stirring to combine.
Bring to a gentle simmer and cook for 15 minutes.
Add cherry tomatoes, spinach, oregano, fish sauce, salt, and pepper.
Simmer for another 5 minutes until the greens are wilted and chicken is cooked through.
Finish with fresh lemon juice, taste, and adjust seasoning.
Serve hot, garnished with fresh herbs.