I've experienced a lot of epiphanies over the years but nothing, and I mean nothing, has ever affected my cooking life like discovering Jim Lahey's no knead bread. Seriously. I've been baking for pretty much my entire life and had tried a million different ways to make artisan style bread at home but nothing really gave me the loaf I was looking for. Then I read an article by Jeffrey Steingarten in Vogue (!) about Lahey's technique. I literally took the magazine into the kitchen and mixed up a bowl of dough immediately. The subsequent bread was a revelation and I've never looked back.
Kenji's prime rib and poached egg techniques are pretty amazing but that bread - nothing will ever touch that bread.
What about you? What changed YOUR cooking life?