Preheat oven to 350 degrees 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 1/2 teaspoon of the salt, mashing or beating to make potato mixture light and fluffy. Stir in 1/2 cup of the cheese.
In a large skillet over medium heat cook beef, sweet red pepper, onion, and garlic until meat is brown. Drain off fat. Stir in undrained tomatoes, corn, pickled jalapenos, cumin, oregano, cayenne pepper (if desired), and remaining 1/4 teaspoon salt. Bring to a simmer; 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, if desired.