Cook ground beef in a large skillet over medium-high heat, stirring until beef crumbles and is no longer pink; drain well. Return cooked beef to skillet.
Stir in tomatoes, next 3 ingredients, and, if desired, jalapeños and liquid. Spoon into a lightly greased 13- x 9-inch baking dish.
Prepare frozen mashed potatoes according to package directions. Stir in 1 cup (4 ounces) shredded Cheddar cheese, sliced green onions, salt, and pepper. Spread evenly over meat mixture in casserole dish, spreading potatoes to edge of dish.
Bake at 350° for 25 minutes; sprinkle 1 cup (4 ounces) shredded Cheddar cheese on top of potatoes. Bake 5 more minutes or until cheese is melted. Let stand 5 minutes before serving.