Is starting an email with "As I told you on Monday" considered condescending?
I agree, it is condescending to start an email with
As I told you on Monday,…
Which is only marginally better than
As I have repeatedly informed you,…
Which is less rude than saying
How many times must you be reminded…?
However, replacing the "I" with “we” and replacing “told” with either “inform“ or “remind” is an improvement,
As we reminded you on Monday,…
The OP's suggested solutions are more diplomatic
- As discussed/mentioned on Monday
I would add a “previously” and also conclude with a “look forward to”
- As previously discussed on Monday…
I look forward to hearing from you and resolving this issue at the earliest.