Preheat oven to 350°F. In a large saucepan cook potatoes in enough lightly salted boiling water to cover for 15 to 20 minutes or until tender; drain. Mash with a potato masher or beat with an electric mixer on low speed. Gradually add sour cream, milk and salt, mashing or beating to make potato mixture light and fluffy. Stir in 1/2 cup of the cheese.

Meanwhile, in a large skillet cook beef, sweet pepper, onion, and garlic over medium heat until brown, stirring to break up meat as it cooks. Drain off any fat. Stir in tomato paste, water, corn, pickles, mustard, and oregano. Bring to boiling; reduce heat. Simmer, uncovered, for 5 minutes to blend flavors. Stir in remaining 1/2 cup cheese.

Spoon beef mixture into a 2 1/2-quart baking dish. Spoon mashed potatoes in mounds over beef mixture. Sprinkle with additional cheese. Bake about 20 minutes or until heated through and cheese melts. Top with green onions and/or bacon, if desired.