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!!
Anchovies! If you're only using anchovies for pizza and putanesca you're missing out on so much! Pop one or two of them in your tomato sauce right around when you fry the garlic before adding the tomatoes. It won't make them taste fishy but it will give it that extra something. Smush them and put them in your glaze! In your dressing!
Also less a single ingredient and more a mindset - booze. You need to use more booze in your cooking. I don't just mean "use more wine", I also mean different types too. Sake in your teriyaki, beer in your beef stew, cider in your pie filling, brandy in your stroganoff, kirch in your (sweet) pie, use it! Istg so many recipes I see online omit the booze or call it optional and it hurts me so much.
Also if you don't have anchovies, you can punch up a sauce in much the same way with fish sauce. Its probably not identical to using actual anchovies but it's much easier to keep in a pantry.