Step 1: Heat olive oil in a large skillet over medium-high heat. Add ground beef and diced onion. Cook, breaking up the beef with a spoon, until browned and the onion is softened, about 6-8 minutes.
Step 2: Drain any excess fat from the skillet. Stir in the minced garlic and cook for another minute until fragrant.
Step 3: Add the taco seasoning and water to the skillet. Bring to a simmer, then reduce heat to low and cook for 5-7 minutes, stirring occasionally, until the sauce has thickened slightly and the meat is well-coated. Remove from heat and set aside.
Step 4: While the beef simmers, prepare the creamy cheese sauce. In a medium saucepan, melt the butter over medium-low heat.
Step 5: Whisk in the all-purpose flour and cook for 1-2 minutes, stirring constantly, to create a roux. Do not let it brown.
Step 6: Gradually whisk in the milk until smooth. Increase heat to medium and bring to a gentle simmer, whisking occasionally, until the sauce thickens, about 3-5 minutes.
Step 7: Reduce heat to low. Add the shredded cheddar and Monterey Jack cheeses, salt, and black pepper. Stir continuously until the cheeses are fully melted and the sauce is smooth and creamy. Keep warm on very low heat, stirring occasionally.
Step 8: Preheat your oven to 325°F (160°C) and warm the hard taco shells according to package instructions, typically 5-7 minutes, until crisp.
Step 9: To assemble, fill each warmed taco shell with a generous spoonful of the seasoned ground beef. Drizzle a generous amount of the creamy cheese sauce over the meat.
Step 10: Top with shredded lettuce, diced tomatoes, sliced jalapeños, and a dollop of sour cream and fresh cilantro, if desired. Serve immediately and enjoy your ultimate weeknight fiesta!