Preheat the oven to 400°F.
Arrange taco shells standing upright in a 9x13 baking dish without overcrowding to ensure even crisping.
Heat olive oil in a large skillet over medium-high heat.
Add diced onions, ground beef, salt, and black pepper to the skillet and cook, breaking up beef, until no pink remains, about 8-10 minutes.
Drain excess grease from the skillet.
Stir in tomato sauce and taco seasoning, cooking for 2-3 minutes until the meat absorbs the sauce but is not too wet.
Fill each taco shell with 1/3 to 1/2 cup of the taco meat mixture, dividing evenly.
Top each filled shell with shredded cheddar-jack cheese evenly.
Bake tacos for 5-7 minutes until the cheese melts and shells are crispy.
Remove from oven and sprinkle with chopped fresh cilantro if desired before serving.