Place all ingredients in a blender or food processor and blend to a thick dressing consistency. Set aside to eat with the tacos.
For the caramelized onion:
Preheat a small skillet on the stove with vegetable cooking oil. Meanwhile, slice the onion into long thin strips.
Add the butter to the pan and add the onion. Stir with a spatula and cook for about 3 minutes. Then add the sugar and salt, and continue cooking until the onion becomes transparent and soft. Save to add to the tacos.
For the salsa cruda:
Cut the onion and tomatoes into small cubes and place in a small bowl. Add the lime juice and salt and mix with a spoon or fork until well blended. Set aside to add to the tacos.
For the avocado sauce:
Cut open the avocado, remove the seed and skin and place in a blender or food processor. Add the rest of the ingredients until you get an avocado sauce, with a slightly runny consistency. You can also do this with a fork in a small bowl. Set aside to add to the tacos.
For the other toppings:
Cut the purple cabbage into thin strips and save for the tacos.
For the beer-battered shrimp and fish:
To make the batter mixture for the fish and shrimp, in a bowl mix the 1 cup of flour, baking powder, dried oregano, hot paprika, black pepper and salt. Then add in the beer and mix with a spoon or a hand whisk.
Preheat the vegetable oil in a deep pan to fry the fish and shrimp over medium heat. Put 2/3 of the flour on a plate, and prepare another plate with kitchen paper to put the fried fish and shrimp there.
Little by little, cover the pieces of fish and shrimp in the flat dish with flour. Then pass through the batter mixture, covering them completely, and then fry them in the oil. The oil is ready to use when a little of the batter mixture forms a lot of bubbles around it. Gradually fry the fish and shrimp (prawns) and place on a plate with kitchen paper to remove excess oil.
To assemble the tacos:
In a large skillet, heat the tortillas and place a little grated cheese in the center of each tortilla and fold in half to make quesadillas. Make as many quesadillas as needed and place them inside a kitchen towel to keep them warm.
To make the tacos, open the quesadillas and put 3 to 5 pieces of fish and shrimp in the middle and add all the toppings and sauces you want on top. Each person can use the toppings they want and make their own tacos.