Preheat oven to 425°F (220°C). Coat a medium baking sheet lightly with non-stick cooking spray; set aside.
In a medium bowl, mix together mashed potatoes and egg until completely combined.
Cut gruyère into 12 long, narrow pieces.
Scoop one heaping tablespoon of potatoes into your hand, then shape into a flattened oval.
Place a piece of the gruyère on top of the potatoes, then gently shape the potatoes around the cheese until it is completely covered.
Repeat with remaining potatoes and cheese. You should have 12 croquettes when you are finished.
Process bread in food processor until you have fine crumbs. Add olive oil, salt, and pepper; pulse a few more times until everything is incorporated.
Place breadcrumbs in a shallow bowl, then roll croquettes in bread crumbs, coating all sides.
Place croquettes on the baking sheet, reshaping if necessary.
Bake the croquettes until heated through and golden brown, about 20 - 25 minutes.
Serve immediately. Enjoy!
Notes
Size - This recipe makes fairly large croquettes. Feel free to cut down the size, either in half or in thirds, if you'd like more, smaller croquettes.Dipping Sauce - The croquettes taste great on their own, but they're delicious with a sauce like marinara, as well.Cheese - I like to use gruyère for the filling, but any cheese that melts well will work. Sharp cheddar is also delicious!Breadcrumbs - Feel free to use panko instead of homemade breadcrumbs.