Slice onion extremely thin and place in a gallon size baggie.
Pour buttermilk over onions, zip baggie shut, and let sit for 30-40 minutes. Make sure onions are fully covered in the buttermilk.
While onions are soaking, heat 1 ½ (might need more) cups of oil in a large, deep frying pan, or a large pan over medium/high heat.
In another large baggie, add flour, garlic salt, pepper, and slap ya mama seasoning, seal and shake to mix.
Remove onions from buttermilk and add to flour mixture.
Shake well to coat.
Gently place onions in hot oil and cook, stirring occasionally, until desired crispiness is obtained (these might need to be done in batches if you do not have a large enough, deep, frying pan).
Place on paper towel lined baking sheet once cooked.
While onion strings are deep frying, heat remaining oil in another frying pan.
Fry tortillas to desired crispiness.
Remove and place on paper towel lined plate until ready to assemble tacos.
Take a couple heaping spoonfuls of beef brisket and add to taco shells, top with onion strings, then squeeze a little barbecue sauce on top of onion strings and top with cilantro.