These days, technical writing is a promising field ( ) you can find a good job.
Not strictly a relative pronoun, yet a relative word that combines well enough with field in this context: where. Here's a similar example from the book How to Get a Good Job After 50: A step-by-step guide to job search success (by Rupert French):
... social media enables you to to more easily build a network of contacts within the field where you are hoping to find employment.
Another one (from: Layoff Survival Guide: Are You About to Be Laid Off or Fired? - by Ade Asefeso):
It doesn't really matter if the job you are going to take part-time participation at is related to the field where you have previously worked.