Preheat oven to 350 degrees F (175 degrees C). Grease a rectangular baking dish large enough to fit the fish fillets in a single layer.
Heat olive oil in a large saucepan over medium heat. Cook and stir onion and garlic until softened, about 5 minutes. Stir in plum tomatoes and red bell pepper; simmer until softened, about 5 minutes. Stir in tomato puree; cook for 3 to 5 minutes.
Stir salt, bouillon cube, oregano, cinnamon, pepper, and bay leaves into the saucepan. Cook until flavors combine, about 10 minutes. Add olives and capers; simmer for 5 minutes. Remove sauce from heat.
Melt butter in a separate skillet over medium heat. Season fish fillets with salt and pepper; cook in the hot butter until browned, 1 to 2 minutes per side.
Lay fish fillets in the baking dish in a single layer; cover with sauce. Arrange sliced peppers on top. Cover baking dish tightly with aluminum foil.
Bake in the preheated oven until fish flakes easily with a fork, 10 to 15 minutes.
