Stir together the chili sauce, soy sauce, and lime juice. Put the tuna steaks in a resealable bag, pour in the chili sauce mixture. Marinate in the refrigerator for at least 10 minutes or up to 2 hours.
Heat a heavy skillet (I use my cast-iron skillet)on high heat, then drizzle in canola oil. Discard the marinade. Add the marinated tuna steaks to the skillet and cook for 4 minutes, until a crust forms on the bottom. Flip and cook for 3 to 4 minutes on the second side.
Plate the tuna steaks and garnish with sesame seeds. Serve immediately.