Heat a large skillet over medium heat and add 1 tablespoon of oil.
Add the diced onion, cooking until it becomes translucent, about 3-4 minutes.
Stir in the minced garlic and ginger, cooking for an additional minute until fragrant.
Add the sliced bell pepper and cook until slightly softened, about 3-4 minutes.
Next, stir in the red curry paste, ensuring it coats the vegetables evenly, and cook for 2 minutes.
Pour in the coconut milk, stirring to combine, and bring the mixture to a gentle simmer.
Add the fish sauce and lime juice, adjusting to taste, then let the curry simmer for 10 minutes.
Add the shrimp and spinach, cooking just until the shrimp turn pink and opaque, about 3-5 minutes.
Taste the curry and adjust seasoning if necessary, then remove from heat.
Serve hot, garnished with fresh cilantro, alongside rice or noodles of choice.