Reducing a tomato based sauce brings out deep “Basso profundo” flavors but diminishes the brighter, fresher notes of the tomato. Adding the un-reduced sauce at the end restores the “treble” flavors back into the mix.
I discovered this trick accidentally when making sauce for two large lasagnas using a pot too small to stir without making a mess, forcing me to set some aside. The result was a thrilling surprise. Returning the lightly cooked sauce near the end restored the bright flavors without masking the deeper ones. It’s like turning up both the bass and the treble on your stereo, thus the “Equalizer”.
A lasagna takes a lot of work – but making 2 is not much worse than 1, and it freezes perfectly. I always double up. Making the sauce ahead of time can break up the effort. A couple of days in the fridge or a few months in the freezer will do the sauce no harm at all.