Heat 3 tbsp. of the oil in a large skillet over high heat. Add beef and cook, breaking up meat with the back of a spoon, for 6–8 minutes. Add onions, bell peppers, cumin, garlic powder, and salt and pepper to taste and cook until vegetables are soft, 2–3 minutes. Add potatoes, then stir in tomato paste and 1 1⁄2 cups water. Scrape up browned bits from bottom of pan. Reduce heat to medium-low and simmer, stirring occasionally and breaking up potatoes with back of spoon, until sauce has evaporated, 25–30 minutes.
Preheat oven to 200°. Pour oil into a wide medium pot to a depth of 3" and heat over medium heat until temperature reaches 350° on a candy thermometer. To make the taco shells, peel plastic wrap off 1 tortilla, then carefully add tortilla to hot oil. Keeping tortilla submerged in the hot oil, use long metal tongs to gently fold tortilla in half so that edges are about 2" apart. Fry tortilla just so until it holds its shape, about 30 seconds, then gently stir taco until blistered all over and edges are golden and crisp, 2 1⁄2–3 minutes more. Transfer taco to paper towels to drain. Repeat process with the remaining tortillas, keeping tacos warm in the oven as you work.
To serve, stuff each taco shell with some of the meat filling, tomatoes, and lettuce, in that order. Serve with pickled jalapeños, if you like.