Preheat oven to 450-degrees. Pour oil into a pan and heat on high.
Once the oil heats up, place the fish into the HOT pan and allow to cook for five minutes. Turn over onto the other side and cook for 4 minutes.
Place pan into the preheated oven for 5 minutes.
Pour in some olive oil into another pan and turn the heat to medium high.
Add shallots, garlic, red pepper flakes and sliced red pepper. Cover with lid and cook over medium heat until lightly browned and soft; about 5-7 minutes.
Remove pan from the oven and place fish on a serving plate, tented with foil.
Place the red pepper mixture into a blender or food processor and add in balsamic vinegar, salt and black pepper. Whizz it all up until super smooth. Taste for seasoning.
Spread the coulis on a plate and serve fish directly on top. Serve with a wedge of lemon if desired.