Preheat the oven to 400 degrees F (200 C).

Heat 1 tablespoon of olive oil in a large skillet over medium-high heat.

Sauté the onions for 3 to 4 minutes, or until they are translucent. Then add the garlic and cook for 30 seconds.

Add the spinach and cook for 30 seconds or until wilted. Remove the pan from the heat and stir in the feta.

Flip the salmon fillets on their side and cut lengthwise through them to create a pocket.

Divide the spinach mixture into four equal parts and stuff each salmon fillet.

Drizzle the fillets with the remaining olive oil and then sprinkle with salt. Bake the fillets on a parchment-lined, rimmed baking sheet for 8 to 10 minutes, or until salmon flakes apart.