This is one of my favorite meals that my mom taught me to make. She taught me to cook, but she seldom used a recipe. So when I ask her how much of an ingredient to use, she often answers "until it looks right"! So I think this is the first time this recipe has been written down. So if it doesn't look right, add a little more of this or a little less of that!
Preheat oven to 400 degrees. Clean potatoes. With a fork, poke each potato 3 times on both sides. Generously rub each potato with olive oil then lightly salt entire potato. Place potatoes in oven directly on middle rack. Bake 45 minutes to an hour, or until tender, turn potatoes over ½ way through baking. Baking the potatoes this way will ensure that the skins are crunchy so they won't tear when stuffed.
After potatoes are done. Remove them from oven and set aside to cool. Reduce oven temperature to 350 degrees.
While potatoes are cooling, in a large skillet brown ground beef and chopped onion on medium heat, breaking ground beef apart into small chunks.
Dice the mushrooms into small chunks. Add mushrooms and garlic to cooked ground beef, salt and pepper the mixture. Cook mixture until mushrooms are tender.
Mix in cream of mushroom soup and approximately a ¼ of the soup can of milk (to rinse out the remaining soup in the can and thin the soup a little. Add a little more milk if the mixture is too thick to stir easily.). Heat to near boiling then reduce heat to simmer.
While the ground beef mixture simmers, slice each potato lengthwise. If possible, try to cut the potatoes so that each half has a flat side so they won't tip over when stuffed.
Carefully spoon out the white of the potatoes and place in a mixing bowl. Don't scrape out all of the white, leave a little so that the skins aren't flimsy. Place the skins in a 9.5 x 13.5-inch baking pan and set aside.
Add the butter and cream cheese to the potatoes and beat with an electric mixer. Slowly add milk until the potatoes are fluffy and creamy. I test for the correct consistency by taking a spoon full of potatoes and turning the spoon sideways. If the potatoes stick to the spoon, then they're done.
Fill each potato skin to the rim with the ground beef mixture. Spoon mashed potatoes on top of each filled potato then top them with the shredded cheese.
Bake uncovered for 20 minutes or until the cheese is melted and the mashed potatoes are hot. Sometimes, just before I remove them from the oven, I broil the potatoes to make the cheese a little crispy on top.