Combine the yeast and warm water in a small bowl and let sit until the yeast starts to dissolve, about 5 minutes.
Place the flour in a large bowl. Mix the yeast and water mixture into the flour with a wooden spoon until smooth. Stir in the sugar, vanilla extract and 1 teaspoon salt, and then cover with plastic wrap, placing the wrap directly on the dough. Set aside in a warm place and let rise for 1 hour.
Bring 6 cups water to a boil in a large pot over high heat. Reduce the heat to low and add the fish. Poach the fish in the simmering liquid until cooked through, 5 to 7 minutes. Remove the fish from the water and set aside in a medium bowl. When the fish has cooled, use a fork to break apart the fish.
Add 2 tablespoons of the olive oil to a medium skillet over medium heat and saute the onions until translucent, 3 to 5 minutes. Add the tomatoes, garlic, 1 teaspoon salt and a pinch of freshly ground pepper and cook for 5 minutes. Add the fish to the skillet and stir to fully incorporate, adding 1 to 2 tablespoons more olive oil if needed to keep the mixture from becoming dry. Cook for another 3 minutes and then turn off the heat.
Heat 2 inches of vegetable oil (about 8 cups) to 375 degrees F in a wide heavy pot over medium-high heat. Working in batches, carefully add the gateau batter to the oil with a 2-ounce ice cream scoop, turning over frequently until browned, about 2 1/2 minutes per batch. Transfer the gateau to paper towels to drain.
Spoon the mackerel into a serving dish and serve with the gateau.
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