Heat the vegetable oil in a large skillet over medium heat. Add the diced onion and sauté for about 5 minutes, until softened.
Stir in the minced garlic and grated ginger, cooking for an additional minute until fragrant.
Add the red curry paste and cook for another minute, stirring to combine.
Pour in the coconut milk, fish sauce, and brown sugar. Stir well and bring to a gentle simmer.
Add the salmon pieces to the skillet and cook for about 5 minutes, until the salmon is just cooked through.
Stir in the sliced bell peppers and snap peas, cooking for an additional 3-4 minutes until the vegetables are tender-crisp.
Remove from heat and stir in the spinach until wilted. Squeeze in the lime juice and mix well.
Serve the curry over cooked rice and garnish with fresh cilantro.