Make the pickled onions up to two days in advance. Thinly slice the red onion into half-moons and place in a jar or small bowl with a sealable lid. Juice the limes and add to the onions, along with the salt. Seal the container, and shake. Store in your cooler.
Prep the guacamole ingredients: mince the jalapeno, garlic, and cilantro. Place in a small bowl and set aside.
Prep the tortillas. Wrap a stack of 8 tortillas in foil and set over your stove burner or campfire, flipping occasionally until they are warmed through. Or, toast each tortilla individually over the stove burner if you would like them to pick up some color. Set aside.
Place the defrosted shrimp (see notes) in a bowl and dust with the Cajun and Old Bay seasonings, tossing so that the shrimp are evenly coated.
Melt the butter in a cast iron skillet over medium high to high heat. As soon as the foaming subsides, add the shrimp. Cook on the first side for 2-3 minutes, then flip and cook until the shrimp are cooked through (1-2 minutes more). Remove from the heat.
As the shrimp are cooking, finish off the guacamole by mashing the avocado with a spoon or fork in the bowl with the rest of the ingredients plus a little bit of the lime juice from the pickled onion jar.
Assemble the tacos. Divide the guacamole and shrimp among the tortillas. Top with the pickled red onions (about a tablespoon per taco). Enjoy!
Defrosting the shrimp: Shrimp can be thawed in your cooler - simply transfer the bag of frozen shrimp from your home freezer to your cooler and allow about 24 hours for it to defrost. If this will be the first meal of your trip, you can start the defrosting process in your fridge at home, then transfer to your cooler right before you leave and cook them that evening.
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