Preheat oven to 250 degrees F.
In a small saute pan, add enough oil so that there is a 1/2 to 1-inch deep layer. Heat oil over medium-high heat until very hot. In the meantime, prepare lettuce, tomato, and avacado; stir about 1/4 tsp of lime zest into sour cream and add enough lime juice to thin sour cream into a "pourable" consistancy; and line a plate with a few paper towels. Test oil by placing a small edge of a tortilla into oil; if it bubbles rapidly, the oil is hot enough.
Once oil is ready, dip a tortilla into the oil using a pair of tongs, and holding it into the oil, let it cook until lightly browned, 30 seconds to 1 minute. Flip tortilla over and fry on the other side. Remove tortilla with tongs, letting excess oil drip back into the pan, then place on paper towels to soak up any extra oil. Place cooked tortilla on a sheet pan and into the pre-heated oven to keep warm while frying the other tortillas. Repeat for all tortillas.