First Principles Thinking
What it is: Break a problem down to its fundamental truths, then reason up from there. Strip away assumptions and conventions.
When I use it: Pricing strategy, architecture decisions, entering new markets. Anytime the answer "because that's how it's always been done" shows up.
Example: When building our stock valuation framework, I didn't start from "what do other screeners do." I started from "what determines a company's value?" — cash flows, growth, risk. Everything else is derived.