Heat one tablespoon of oil in a large pan over medium-high heat. Add the onions and sauté for about 5 minutes, until they turn translucent. Remove the onions from the pan and place in a large bowl.
1 medium onion
Add the potatoes to the pan and sauté until they're nicely browned. (Unless you've got a really large skillet, you'll need to do this in batches.) Add the potatoes to the bowl with the onions.
8 cups potatoes
Add the rest of the ingredients to the bowl and mix well.
1 pound ham, 16 ounces sharp Cheddar cheese, 1 cup plain Greek yogurt, 2 tablespoons Worcestershire sauce, 2 tablespoons apple cider vinegar, 1 tablespoon Dijon mustard, 1 teaspoon salt, ½ teaspoon black pepper
Pour the mixture into a 9x13 baking pan and bake for 30 minutes.
If you want even more cheese, remove the pan from the oven, sprinkle more cheese on top, return to the oven, and bake 5 to 10 minutes until the cheese is melty and bubbly.