How do you track when you have no idea?

I agree to move on and let it go… but I totally get it. What I would do if it were me… I would find the closest quick add food item and guess on how much I ate based on the recommended serving for that item… I usually pick the least healthy option from quick add as well when I don’t know what the restaurant uses to cook (butter, veg oil, olive oil).

While losing, I did my best to keep track of what I ate (as you have) and log it after. That was one time when I would use other people's recipe/dish calorie entries in the database most of the time (i.e., log "cheese lasagna" rather than deconstructing into X amount of pasta, Y amount of sauce, etc. I'd make my best guess at portion sizes. (The more I weighed foods at home, the better my size estimates got. It's OK to have a learning curve . ) I'd use a medium to high calorie example of the food from the database, not a lowball one or one that was high to the point of being an outlier. I'd just log it and go on, not worry over it: Worrying burns no extra calories and feels icky.

I'd point out that the calorie deficit through the week usually means there's still fat loss for the week, even with a high day, as long as not crazy high.

If it was polite and I could do it discretely, I'd snap a photo of the food with my phone camera (no flash!) to help me remember what I ate, and estimate portions. If it was a photo of my plate, I'd include a fork or something as a gauge of size when I logged later.

That worked fine, for me.

As time has gone on, I've gotten more relaxed (or more sloppy). I don't even always log that sort of thing, as long as it's rare. (I'm in year 8 of maintenance at this point.) One rare meal really is a drop in the ocean of life. I just watch the scale and clothes-fit, and cut back if weight creeps up in maintenance. My routine daily calorie goal is a very small bit below my actual estimated maintenance (by 100-150 calories, maybe), which creates a little wiggle room for extra indulgence once a week or so.

During loss, I'd plan days around known special eating occasions: Maybe banking a manageable few extra calories for a few days in advance of the event, certainly eating lightly during other meals that day, maybe doing some manageable extra exercise that sounded fun. (I don't generally think of exercise's point as "buying calories".)

Realistically, we can't pin down calories for a meal like that. Not eating like that (ever) is not something I'm willing to do, either, because good life balance matters. As long as it's rare, I think the thing to do is estimate, let it go, and see how the scale settles out after a week or so.

Managing my mindset around this sort of thing was part of learning how to better manage my weight, too.

(NB: After a meal like that, I'd expect a scale jump that could last anything from a few days to a couple of weeks, maybe longer in women with monthly cycles. Usually restaurant meals have more sodium and carbs than our standard weight-loss routine, and that will add water weight while those are digested/metabolized, not to mention the mass of food residue that will eventually exit as waste.)

Best wishes!