Make the tzatziki sauce ahead of time by adding all of the ingredients to a bowl, and stir well. Cover and refrigerate until you are ready to use it on the potato skins. This makes a bunch, so you can use it on sandwiches, or make traditional gyros as well and use it on them.
To make the gyros, start by getting some plastic wrap ready, as you will want a large enough piece on both of the bottom and top of the meat. Lay the plastic wrap on your meat board, then top with your lamb chops, adding the top piece of the plastic wrap on top. Use your meat mallet and pound the meat into approximately 1/4 inch thick, being careful not to tear the meat. Best practice is to pound from the center outward. Repeat this process with the boneless beef round.
Now, if you have a spice mill, or better yet, a coffee grinder dedicated to spices, get it out. Add in all of your spices, and grind until you have a fine powder. To the pounded lamb, add about a tablespoon of olive oil to the top, and spread evenly over the top. This will act as a glue to hold on the spices we just ground up. Use about half of the spice mix and evenly coat the top of the lamb. Use a fork, and quickly jab the meat, making certain that all of the spice flavors penetrate the meat.
Do the same with the beef, reserving about a tablespoon of the spice rub.
Now, cut about six inch strips of the kitchen string, as we are going to roll up the meat and tie it to prevent it from unraveling. Add the lamb on top of the beef, and tightly roll into a log. Secure and tie it with the string. Rub more olive oil on the top of the meat, and use the remaining herb mixture to coat the meat log. Cover and place in the refrigerator to let it marinate for at least an hour, or overnight.
Before you are ready to grill, remove the meat from the refrigerator and let it come to room temperature before placing it on the grill.
Bring your grill to a medium heat, and place the meat log on the grill, cooking for at least 1 hour. Check the meat about every 10-15 minutes, turning a quarter turn, building a nice dark crust on the meat. The meat should register about 140 degrees before removing it from the grill.
Cover and let the meat rest for about 10 minutes before slicing into it and making your gyros sandwich. Slice the meat when you are ready.
Now, make the potato skins.
Begin by preheating your oven to 400 degrees. Poke the top of each potato with a fork and place on a baking sheet. Cook in the oven for 1 hour. Once cooked, remove from the baking sheet, and place on a plate to let cool for about 15 minutes.
Once your potatoes are cooled, use a sharp bread knife and you will cut lengthwise. Be gentle and scoop out the potato, leaving a bit so the skin holds up. You still want some potato in the shell.
Brush the interior and exterior of each potato skin with the melted butter. Season the inside of each skin with salt and pepper.
Place them under the broiler for 12-15 minutes until the insides get a bit crispy from the melted butter. I like to go a bit extra crispy with these, but it's your call. Pull them out when you know they are ready.
Now it is time to assemble the potato skins. Slice a generous amount of the gyro meat. Place inside of each potato skin, then top with onions, tomatoes, and the tzatziki sauce. Plate, and serve.