Hello 저예요 Both could be correct, but to make it even more natural you can just say - The thing I told/talked to you about. You can leave out 'which' or 'that' because they refer to the object of the verb (The thing that usually comes after the verb) - I talked to you about the thing! Hope this helps

Hi - both are okay. Don't worry about taking the "that" out, in my opinion. I know LOADS of natural English speakers that would include "that" every time that they used this phrase (including me, in fact). "Talked" suggests to me that you had a longer conversation about the thing. "Told" suggests to me it was just a quick exchange of words. Tom