Take the cream cheese out of the fridge at least 30 minutes prior to using or microwave it for 20-30 seconds to soften it up. Boil a large, salted pot of water for the shells. Cook them for 1 minute less than package directions indicate.
Add the olive oil and onion to a skillet over medium-high heat. Sauté for 5 minutes.
Meanwhile, Preheat the oven to 375F and move the rack to the top third of the oven. Spread the marinara sauce in an even layer on the bottom of a greased 9x13 baking dish.
Stir in the garlic, Italian seasoning, and ground beef to the skillet. Cook until the beef is browned and cooked through, breaking up the beef with your spoon as you go along (about 8 minutes). You shouldn't have to drain the fat if using lean ground beef, but if there is some, spoon out most of the excess fat.
Once the beef is cooked, reduce the heat to low and stir in the cream cheese until it's incorporated. Give it a taste and season with salt & pepper as needed.
Drain the shells and let them cool for a few minutes. Once they're cool enough to handle, use a small spoon (I used a teaspoon) to stuff the shells with the beef mixture. Add them to the baking dish as you go along.
Once all the shells are stuffed, evenly sprinkle the cheddar over them. Bake uncovered for about 15-20 minutes or until they're hot and the cheese has melted. I then broil for a few minutes to brown the cheese. Note: you can cover the baking dish with foil prior to baking if you wish. I like to have some crispy bits, but if you want it all to be softer, do that (take the foil off to broil, though).