Heat oven to 400°F. Line a rimmed baking sheet with parchment.
Wilt spinach in the microwave for 60–90 seconds. Squeeze out excess moisture in a towel.
Stir cream cheese, wilted spinach, crab, Parmesan, garlic, dill, lemon juice, red pepper flakes, and a pinch of salt and pepper.
Pat salmon dry. Cut a deep slit along the side of each fillet to create a pocket.
Rub salmon with olive oil. Season all over with salt, pepper, paprika, and lemon zest.
Divide the filling among the fillets, gently packing it into each pocket.
Arrange salmon skin-side down on the prepared sheet. Brush tops with melted butter.
Bake 15–18 minutes until salmon flakes easily and reaches 125–130°F in the thickest part.
Broil 1–2 minutes for extra color, if desired. Rest 3 minutes before serving.