Prepare a large platter lined with paper towels. Pour enough vegetable oil to measure 1-inch deep in a large skillet. Place skillet over medium heat and clip a candy thermometer to the inside. Heat oil until it reaches 365ºF. Using tongs, place a tortilla in oil and cook for 15 seconds. Flip tortilla and fold in half, using tongs. Cook until crisp, about 15 seconds longer. Drain on platter and sprinkle with salt. Repeat with remaining tortillas, checking that oil returns to 365ºF before making a new shell.
In a pot over medium heat, warm olive oil. Sauté onions until softened but not browned, about 10 minutes. Stir in garlic, jalapeño and chili powder; cook until fragrant, about 1 minute longer. Add beef and cook, stirring to break up clumps, until nearly done, about 10 minutes. Pour in tomatoes with juice, season with salt and pepper and cook, stirring often, until most of liquid has evaporated, 20 to 25 minutes. Season with more salt and pepper, if desired.
Carefully spoon some of filling into each shell and serve warm.