In a large skillet, heat the vegetable oil over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes.
Stir in the minced garlic and grated ginger, cooking for an additional 1-2 minutes until fragrant.
Add the red curry paste to the skillet, stirring well to combine with the onion mixture. Cook for another minute.
Pour in the coconut milk, fish sauce, and brown sugar. Stir until everything is well mixed and bring to a simmer.
Add the fish chunks to the skillet, gently stirring to coat them in the sauce. Cook for about 5-7 minutes, or until the fish is cooked through and flakes easily with a fork.
Stir in the cherry tomatoes and spinach leaves, cooking for another 2-3 minutes until the spinach is wilted.
Remove from heat and squeeze lime juice over the curry. Season with salt and pepper to taste.
Serve hot, garnished with fresh cilantro.