Let it play out - if they come out of their shells and RP, they'll make it interesting. And if they save the day w/o a fight then you've got something in your library of encounters to pull out some other time when in need.

As for how to run to cookout: Let them figure out party logistics, but be ready to be an NPC or two that jump in with suggestions. Think through the things they'll need (food, drinks, guests) ahead of time and come up with some options if they ask the right questions. Maybe someone knows of a local pig farmer who'll trade some pork for an IOU from he party.

Then think about how Teostra may respond. If they make friends then maybe Teostra reveals that he has been talked into hunting the party because ____. That can set you up for your next adventure. Or two adventures, counting the pig farmer.

I think your players have an awesome idea, and it would be a good idea to lean into it. However, you took the time to prepare the encounter, and as the DM you're playing the game too and you all deserve to have fun together.

Now we're talking about cooking a meal fit for a force of nature. You're not going to be satisfied with a simple sandwich or a grilled steak. No, you're a dragon, your palate deserves to be graced with meals that surpass those given to kings of men.

Here's a couple ways I could see that playing out

• Teostra wants some exotic meal made with ingredients that are rare / magical / dangerous to procure
• A dragon eats its kills, and has a taste for monsters. The wilder and stronger the better
• The dragon is presented with a meal but is unaccustomed to the seasonings. Maybe it has an allergy, or it's too spicy, and this enrages it. Then you still get to have your fight with Teostra while the party tries to calm this fit

In another comment you said that the dragon was motivated to destroy the village because it's in the way and it wants to see something new out of curiosity. Even if the party gives it a fantastic meal, these things are ephemeral and I think Teostra will have wanderlust again at some point. So I also wouldn't think that cooking a meal would be a one-and-done situation, and maybe they'll have to continually parley with the dragon. That in itself might be amusing enough for a dragon not to destroy all humans, but hey, it's a dragon so maybe not

Personally I'd probably have something else (Nergigante or Kushala maybe) crash the party and make Teostra into a temporary ally, then if the fight goes well make him a permanent one. He'd never join the party for real, but there would always be the option to have him jump in for the occasional big dramatic set piece.

Awesome players thinking outside the box. For setting the scene, your players are the ones rubbing the cookout, right? Sounds like a them problem.