Preheat oven to 350 degrees. Spray 9x13 inch baking dish with nonstick cooking spray.

Bring a large pot of water to boil. Cook the shells according to package directions and drain well.

Meanwhile, brown the ground beef over medium-high heat in a large skillet. When it is about halfway through the browning process, add the onion and continue cooking until the beef is browned and the onions are soft. If needed, drain any excess grease from the skillet.

Add the taco seasoning and water to the ground beef mixture. Simmer for 10 minutes. Add the salsa and stir to combine. Remove from the heat and let it cool for a few minutes. Once cooled, stir in half of each cheese.

Spoon the seasoned ground beef mixture into the shells and lay them in a single layer in the prepared dish. Top with the remaining cheese, Rotel tomatoes, and minced jalapeno. Bake covered for 20 minutes. Uncover and bake for an additional 5 minutes. Sprinkle with minced jalapeno or green onion. For best results, serve promptly.