Grilled oysters are delicious and simple and somewhat primeval: humans have been eating oysters cooked over a fire for a long, long time. They're a staple at Hood Canal get-togethers: Usually they're cooked over an open fire, usually it's raining, and frequently the oysters are clusters just plucked from the closest beach.
Tip 1: Turn down the heat
The only way you can really mess up a grilled oyster is to have the heat set too high. On a propane grill, keep the direct heat to medium / medium high, and consider cooking the oysters over indirect heat with the grill lid down so that it turns into an oven. Over an open fire or briquets the rule of thumb is that if you hear the shells popping, the heat is too hot.
Tip 2: Protect yourself
Growing up, we'd just throw the oysters on the grill whole, often cooking clusters instead of singles. If you're cooking oysters this way, be double dog certain that your heat isn't too hot, and consider pointing the hinge towards you. As the oysters steam open, they may pop and explode, and having the hinge towards you helps direct the oyster shell shrapnel in the opposite direction. (If they're popping and exploding, it's a sign your heat is too high, per Tip #1).
If you're taking the tops off the oysters before you put them on the grill, it's a bit less dangerous because there will be less pressure inside the oyster. But they can still explode, so either way: you might consider wearing sunglasses! Also be sure to procure a waterproof and heatproof glove, or handle them with tongs, because the liquid inside will be scalding hot.
If you're cooking oysters during vibrio season (when the water is warm) be sure to cook them to 140 degrees for 15 seconds to be sure they're safe to eat.
Tip 3: Butter
There are [at least] two things to consider when you're thinking about butter. Do you add it before the oyster is cooked, or after?
The classic way to serve barbecued oysters is drizzled with garlic butter and splashed with Tabasco. This method involves cooking the oysters whole. After about 15 minutes or so on a grill, the oysters will steam open about a quarter inch. Pry the shells apart with an oyster knife, and add a spoonful of sauce. You can eat it right away, or put the oyster back on the grill (cupped side down) to let the butter cook a bit longer. We have also been known to simply shuck the cooked oysters into a vat of melted garlic butter for hot holding and max butter factor.
A different method is to take the top off the oyster before you cook it, add the butter, and then roast the oyster cupped side down. This has the advantage of reducing the burn factor (since you're not handling a hot shell full of hot oyster liquid) and allowing the oyster and butter to caramelize together. It has the disadvantage of increasing the likelihood that a bunch of melted butter is going to drip out of the oyster and into the bottom of the propane grill, where it will flame up and cause greasy smoke.
We love Renee Erickson's "snail butter" (available in her cookbook), Hog Island Oyster Company's chipotle-bourbon oyster butter (which we sell online), and our own nettle clam steaming butter (available here).
Tip 4: Improvise
Don't have a grill? Put the pre-shucked oysters on a cookie tray, using rock salt to keep them upright, and bake them at 400 degrees for 8 to 10 minutes.
Tip 5: Start Cupped Side Up?
For decades we've grilled oysters cupped side down, and told our customers to do the same. This is still the best (and only) way to put the oyster on the grill if you're starting with a topped off oyster.
However, in the summer of 2019 we made an amazing discovery: when you grill an oyster whole and start it on its flat shell, the adductor muscle releases more easily from the top shell. Once the oyster opens up, you can easily remove the top shell, add your butter, and finish cooking... this time cupped side down. Because the shell gets direct heat on both sides, the oyster releases easily from both the top and bottom shells.
Grilling oysters with the cupped side up. These are beach grown hama hama oysters that we picked just before cooking, hence the variety in sizes and the super fluted shells.
Once the oysters steam open, remove the top shell and put the oyster back on the fire cupped side down with a bit of garlic butter in the shell. Cook until the butter starts to carmelize.