This is a variation on the classic Mexican aguachile, which has a green sauce. I use scallops here, but shrimp, fish, clams or oysters would all be good options.
Heat a comal or griddle or cast iron skillet over high heat. When its hot, char the onion and the garlic clove well. When both cut sides of the onion quarters are blackened, roughly chop and put unto a blender. Peel the garlic and put the clove in the blender.
While the onion is charring, mix in some salt with the sliced red onion and toss with about 1/4 cup of the lime juice. Let this sit in a bowl until you are ready to serve. Doing this quick pickles the onion and removes any sulfur bite to it.
Add the rest of the lime juice, Maggi, soy, about 1 teaspoon black pepper, the black garlic cloves, if using, and the water to the blender and puree everything.
Toss the scallops and cucumber with the sauce and put some on everyone's plates. Add the avocado, the red onions, the cilantro and crushed chiltepin chiles. Serve at once.
Notes
You can find black garlic in some supermarkets, Trader joe's tends to carry it, or you can buy black garlic online. Or just skip it.
Keys to Success
Use high quality seafood. Sushi grade. Make sure you read the headnotes for food safety issues with raw seafood.
If you can't find the dried chiltepin or chile pequins, you can use any crushed hot pepper, or you can add serranos or some other fresh hot chile to the blender.
Serve this with tostadas or tortilla chips ideally, but you could also eat it by itself.
Really blacken the cut sides of the quartered onion. It makes a difference in flavor.
If you happen to have Yucatecan recado negro around, unlikely but possible, add a couple spoonfuls of that to the blender. You'll thank me later.