Preheat oven to 350°F. Grease a 3-quart baking dish; set aside.
In a large (12-14 inch) skillet over medium-high heat, cook the ground beef until no longer pink. Transfer the beef to a large bowl; drain excess grease from the skillet.
In the same skillet, heat 2 tablespoons of oil over medium-high heat. Add half of the frozen potatoes in an even layer. Cook for 5-7 minutes, until lightly browned. Season with a little bit of salt, to taste, turn the potatoes, and cook for 3-4 more minutes, until lightly browned on the other side. Spoon into the bowl with the beef.
Repeat the procedure with 2 more tablespoons of oil and remaining potatoes. Transfer to the bowl.
Stir the condensed soup, milk, mustard, pimentos, Pepper Jack cheese, green onions, garlic powder, ½ teaspoon salt, and pepper into the bowl with the beef and potatoes. Mix gently until well combined. Taste and season with additional salt and pepper, if desired.
Spoon the beef and potato mixture into the prepared dish. Sprinkle cheddar cheese on top.
Bake until the casserole is heated through and the cheese is melted, about 30 minutes.
Let the dish rest for about 5 minutes before serving. Garnish with additional chopped green onions or fresh herbs, if desired.