1. Place the sesame seeds in a cold, dry skillet; toast over medium-high heat, stirring constantly, until they become aromatic and lightly colored.
2. Combine half the seeds with the flour and salt on a plate: dredge the fish in the mixture.
3. Heat the oil in a large nonstick skillet over medium-high heat; add the salmon and cook until browned, about 5 minutes per side. Remove from pan and keep warm.
4. Add the spinach to the skillet and cook, tossing constantly, until wilted; mix in remaining sesame seeds. Serve.