Place the cinnamon on a small plate. Dredge the tuna in the cinnamon to coat.
Heat the oil in a nonstick frying pan over medium-high heat. Sear each side of the tuna to just brown the outside, 1 to 2 minutes per side, 4 to 8 minutes total. Slice with a sharp knife into 1/2-inch-thick pieces, season with salt, and serve.