This isn't a real Thai recipe. There's a real Thai corn and crab soup, Kaeng Poo Kab Kao Phod, but the recipes I found for it call for a can of creamed corn. I went a different way.
First up, I needed a base for the soup and this seemed a good time to make a batch of shrimp stock. Every time I cook shrimp, I keep the shells and I had accumulated a quart bag full in the freezer. I knew it was going to make more than what I needed today so I kept the seasonings simple:
1 quart shrimp shells
2 corn cobs
1 half onion, cut in two
1 teaspoon salt
1 teaspoon whole peppercorns
All that went into a large pot with water to cover--around six cups--and simmered for twenty minutes. Then I strained it, set two cups aside to freeze, and started into the dish proper.
To start building the Thai flavors, I added:
2 lemongrass stalks, trimmed and crushed
1 hot pepper, split
2 cloves garlic
1 piece dried galangal (Galangal is a relative of ginger with a less sharp, more floral flavor)
4 ears of corn kernels
and simmered for a half hour.
Afterwards, I fished out the lemongrass, galangal and a cup of the corn, added:
1 cup coconut milk
1 Tablespoon fish sauce
and blended in batches until the soup was fairly smooth.
Then I returned the reserved corn and added:
6 ounces of picked crab (Lump crab would have been better. Crab claws would have been better still. Dropping in a whole fresh crab might have been interesting.)
and, if I had thought of it, this would have been a good time to add some thinly sliced kaffir lime leaves. But I didn't remember until much later so I only added them to the leftovers.
and simmered for 5 minutes to blend the flavors.
Finally, I garnished with copious cilantro and scallion, a squirt of sriracha and a squeeze of lime. And, after tasting, a bit more fish sauce and, to compensate for few-day-old corn, a bit of sugar.
I think you can see that the texture ended up kind of sludgy. The fresh corn was kind of tough and didn't blend so well instead of the creamy result you'd get from blending canned or frozen corn. You're going to get sludge from the picked crab anyway so that's OK.
The corn flavor, once I had tweaked it with a little sugar, was strong through and harmonized nicely with the crab. The lemongrass and galangal flavors, which were prominent before blending were kind of lost and the coconut was pretty mild so it was up to the herbs and the funkiness of the fish sauce and kaffir lime leaves to add complexity to the soup and make it definitely Thai. A slight shift and this could have easily ended up Chinese or Southwestern or a bisque and been just as good. Lots of room for variation to preferences here.