How can I use "fill someone in" passive voice?
Solution 1:
That's almost right, but the wording sounds a little clunky. It sounds like there might be an "about" missing, since "filled in" isn't a direct substitute of "told" in all cases. For example, "What did you tell him?" cannot be replaced by "What did you fill him in?" - it would have to be "What did you fill him in on?" or "What did you fill him in about?"
The phrasing "what I was filled in" is an odd-sounding subject for the sentence, it would be better to include "about" - What I was filled in about today is that we are planning a big project. Of course, that wording is a bit clunky as well, so you should probably just say:
I was filled in today that we are planning a big project.
Solution 2:
You need the phrasal-prepositional verb* fill in on here:
Active: The company filled me in on our plans for a big project.
Passive: I was filled in on our plans for a big project.
Your active version: What the company filled me in on today is that we are planning a big project!
Your passive version: What I was filled in on today is that we are planning a big project!
You could use the preposition about instead of on. But to follow filled in with a that-clause is not idiomatic usage. Here are examples from the Corpus of Contemporary American English:
filled [pronoun] in on (179)
filled [pronoun] in about (16)
filled [pronoun] in that (1)
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