In a blender, combine carrot, ginger, scallion whites, vinegar, sesame oil, tamari, 3 tablespoons canola oil, and 1 tablespoon water. Blend until smooth, scraping down sides of blender jar as necessary (add up to 1 tablespoon more water, if necessary, to adjust consistency). Season with salt.
Place sesame seeds on a rimmed plate. Season tuna steaks with salt, then press both sides of the steaks into seeds to coat.
Heat remaining tablespoon canola oil in a large skillet over medium-high heat. Cook fish in skillet 1 to 2 minutes per side for medium-rare (if steaks are particularly large, cook them in two batches).
Serve fish topped with sauce and sprinkled with reserved scallion greens.