A sauce might be lovely and shiny and perfectly emulsified, but taste flat. Inexperienced cooks are tempted to add something fancy or complicated in this case, but it usually comes down to balance. Acidity, sweetness, fat, and salt are all counterweights to each other, and an absence of one makes all the other flavors dull. So instead of adding “a pinch of this” and “a dash of that,” just take a taste of the sauce on its own (with a fresh spoon), and then ask yourself if it tastes bright, too rich, flat, or just a little bitter. That will give you some indication of where you should go.
Heat a little of the sauce in a separate pan to avoid messing up your whole batch. Add a pinch of salt and stir, and then wait a few seconds to taste again. Salt doesn’t make your sauce taste salty; it just clarifies flavors. If the sauce seems thick, add a small squeeze of an acid like lemon juice or vinegar. There’s a valuable fifteen-minute drill to be done dividing a sauce into three small bowls and seasoning them differently, then comparing the sauces side by side to see how small differences affect the end result.
Overseasoning with one ingredient too quickly (most commonly salt) is a classic noob move. If you over season you’re going to be doing a lot of useless adding. Similarly, a lot of people will add an ingredient and immediately taste it without letting it come to a simmer or at least stirring for ten seconds or so. Allow the flavor to meld. Similarly, if you add a squeeze of lemon juice to a soup and you find it too acidic don’t add a cup of sugar, add a pat of butter or a little bit of olive oil to round out the flavor.
If it tastes almost right but not quite there, then think about texture and temperature. If the sauce is too thin it will taste insipid, even if it’s properly seasoned; if it’s too thick it will suppress its brightness. Try reducing it a bit — that will enhance the flavors without introducing new ones. And if you’re tasting any bitterness, it might be from the browned bits you got at the outset. That’s easy enough to correct — strain the sauce or thin it out with a bit more stock.
Try this exercise with other sauces as well, and soon you will have a baseline for balanced flavor. After a while, you won’t need to measure; you’ll know exactly how much of this or that to add. You don’t need to commit anything to memory to “fix” a sauce like this, you just need to taste, to notice where the flavor is falling short, and know how to fill in the blanks. With practice, this will become a valuable lesson every time you step up to the stovetop.