Tell me about that one ingredient, that when discovered, it opened your mind to a whole new world of flavor possibilities!
For me, the first to come to mind is Worcestershire sauce. I'm talking about way back in my youth. It was my first introduction to what we now call umami. When I noticed my mom put it in her meatloaf I began experimenting. At the time I was just blown away by how much it changed things. I even used to put it in my Top Ramen I was so obsessed lol. More recently, dukkah. Trader Joe's is correct when they say to take bread, dip it in olive oil, and then dukkah. So tasty!!
Chicken. Perhaps it is less the ingredient in this case than the techniques associated.
It's not that I didn't each chicken, but learning to roast a whole chicken was the gateway to real day-to-day cooking for me. I'd roast a chicken on the weekend, make sandwiches or salad with leftovers, make broth with the carcass, make soup or pot pie or gravy with broth and other leftovers, etc. Before cooking was something that seemed like a special occasion but now I had something that sort of begot more food naturally.
If I had to pick a single ingredient, probably cumin or curry spice if I can be allowed to cheat. So many dishes to add that to.
Butterflied roast chicken was the single recipe from Salt, Fat, Acid, Heat that really nailed home so many of the lessons for me - especially about seasoning your meat overnight! Amazing what a difference it makes - one of my fave easy cooks now!