Translation:Begin to eat!
Perhaps in the sense that nuqneH? functions as the Klingon counterpart to "Hello".
If someone comes up to you and doesn't immediately state their business, you might prompt them with nuqneH? "What do you want?" So it's a conversation starter, but not a ritualised greeting like "Hello" is that would be routinely used.
Similarly, if you're sitting together for a meal and the others are just staring blankly into space, you might say peSopchoH! "Start eating!", but I doubt that it's a ritualised wish like Bon appétit that is routinely used at every meal.
That said, you may be aware of the Conversational Klingon tape (later CD), which includes a scene with a Terran in a Klingon restaurant.
There, after the waiter has brought the Terran some food and collected the money, he says peSop, tera'ngan which the narrator translates as "Bon appétit, Terran."
The peSop is almost certainly a mistake for yISop (there is only one Terran tourist in that scene), so in a way, yISop or peSopchoH or similar is a canon phrase for the beginning of a meal. But I wouldn't read too much into it -- my opinion is that Klingons wouldn't routinely say it the way many Terrans would use Bon appétit (or "thank you" or "hello" or similar phrases).
peSopchoHtaH is certainly a fine instruction to give and a grammatical sentence. The -taH implies that you are talking about something that will be going on for a while and with the -choH says that this is the beginning of that ongoing eating. Without the -taH it refers only to the start of the eating and neither implies nor denies that the eating will be ongoing. You might be talking about eating a single quickly consumed scrap or a large multi course meal, but either way, you're talking about when the person goes from not doing that activity to doing that activity. Since eating usually is a process that goes on for a little while, it is almost always going to be appropriate to include the -taH, but it is also OK to leave it off if all you really want to focus on is the start and not the ongoing process.
How to properly translate that into English is a whole other discussion. The English translation "Start eating!" definitely seems to include an implication of ongoing eating and this course almost always uses the -taH suffix with the "-ing" ending. So I've changed the "best translation" to "Begin to eat!" which really means the same thing, but has less focus on the ongoing action and no "-ing" ending so it matches with the absence of the -taH suffix.