Bring to a boil the vinegar, water, and a 1⁄4 teaspoon salt. If you want, you can also add in some black peppercorns, whole cumin seeds or dried oregano. Add the onions and simmer for 3 minutes. Remove from the heat and let cool. The onions will keep in an airtight container in the fridge for three weeks. Next, make the cilantro yogurt sauce by blending the yogurt, lime juice and cilantro in a blender until incorporated (if you don’t have a blender or food processor, you can finely mince the cilantro and mix it with the yogurt and lime juice). Add salt to taste.
Heat the olive oil in a large nonstick pan over medium-high heat until very hot but not smoking. Sprinkle the black bass with salt and pepper and cook, skin side down, gently pressing down with a spatula to ensure contact with the pan. Cook until the skin is browned and crisp and the flesh is almost opaque, 6–8 minutes. Flip and cook until the flesh is opaque throughout, about 1 minute longer. Set aside to rest and warm the tortillas. Set a large pan over medium-low heat. Arrange 2-3 tortillas in a single layer. Flip them after 30 seconds or so. The tortillas should be very warm and become softer, but don’t let them brown. As they come off the stove, wrap them in a clean tea towel. Repeat with a second batch. Remove the skin from the fish and discard it. Portion the fish into taco-size pieces and serve inside the tortillas (double up on tortillas for an extra sturdy taco) and top with the cilantro yogurt sauce, pickled red onions, and extra cilantro leaves.