How to Make It
Combine onion, vinegar, water, sugar, and 1/4 teaspoon of the salt in a small saucepan over high; bring to a boil. Let boil 1 minute. Remove from heat, and let stand until ready to serve.
Stir together yogurt, sour cream, lime juice, cumin, and 1/4 teaspoon of the salt in a small bowl. Set aside.
Whisk together eggs, milk, pepper, and remaining 1/4 teaspoon salt in a medium bowl until combined and slightly frothy.
Melt butter in a large nonstick skillet over medium. Add egg mixture to hot skillet, and cook, stirring constantly with a spatula, until eggs are set, 3 to 4 minutes. Transfer scrambled eggs to a serving bowl; cover to keep warm until ready to serve.
Arrange eggs, tortillas, pico de gallo, cilantro, Cotija cheese, avocado, drained pickled onions, and yogurt sauce on a serving tray. Assemble tacos as desired.