Once you’ve figured out what you want to specialize in (and make no mistake, in a larger company you’ll have a narrower role than in a startup), write your resume around those skills. Market yourself (write a blog, try to speak at conferences, etc) as an expert in that topic. Identify what shortcomings you still have and work on them through side projects, reading books, taking an online course, etc. When you get to the interview stage and people point out your lack of expertise, explain (better yet, demonstrate through relevant examples) that working at a startup has taught you to pick up new skills quickly. Convince your future manager that it’s in her benefit to recruit someone who knows how to learn new skills rather than a one-trick poney, even if the poney is very good at its one trick.