Food writer and Eater’s editor-at-large Helen Rosner has the simplest secret-ingredient tip in all of cookendom:
Transforming a dish with one new ingredient feels magical, like you’re the Ratatouille rat chomping on a big mouthful of cheese and strawberry. It inspires some people to carry around hot sauce or a proprietary salt mix. It’s especially revelatory to those of us who grew up on the bland flavors of the midwest.
Other universal add-ons include:
But above all, salt and lemon. In the cookbook Salt Fat Acid Heat, Samin Nosrat writes, “Salt has a greater impact on flavor than any other ingredient.” She talks about a chef who added three heaping palmfuls of salt to Nosrat’s polenta. She was terrified, but she tasted it: “The corn was somehow sweeter, the butter richer. All of the flavors were more pronounced... No matter how I tried, the word salty did not apply.”
Nosrat’s favored acid is vinegar; she has a similar story of adding vinegar to a carrot soup, afraid it would turn into “a sweet-and-sour abomination,” but finding it brought out the flavors of “the butter and oil, the onions and stock, even the sugar and minerals within the carrots.” Still, she makes room for “a hit of acid at the very last seconds” from a freshly squeezed lemon, lime, or orange. “The volatile aromatic molecules disperse over time,” which is why the lemon in a restaurant entrée is served on the plate for you to squeeze yourself.
Rosner recommends fresh lemon in her excellent soup recipe called “Roberto”, which is written with inexperienced cooks in mind and has descriptions like “[The sausage] should look speckled with dark spots, like a leopard or a cute dog.”