Heat vegetable oil in a large pan over medium heat. Pat tilapia loins dry with paper towels. Pan fry the tilapia in a single layer (do not crowd the pan), 7-8 minutes per side, until both sides have a golden crust. Remove onto a separate plate.
Add the sesame oil, garlic and ginger to the pan. Once the garlic and ginger start to sizzle, add the mirin and soy sauce. Reduce heat to low.
Put the fish back into the pan and simmer, spooning sauce over the fish, 3-5 minutes.
Divide the cooked rice evenly among serving plates. Add the spinach to the pan and cook 1 minute, until spinach wilts. Plate the fish and spinach over the rice. Garnish with red pepper flakes, if desired.