Bring a pot of salted water to a boil. Add potatoes and cook until tender but still firm, about 15 minutes; drain.
Mash the potatoes using a potato masher or mixer. Add Monterrey Jack cheese, butter, sour cream, salt, and pepper and mix well.
Whip the potato mixture until smooth, pour into a greased 2 quart casserole dishes.
NOTE: At this time the casserole can be covered and refrigerated until ready to bake. (This casserole is a great prepare ahead dish) Remove from fridge about 1 hour before serving time. Let sit to come to room temperature for about 30 minutes
Bake the mashed potato casserole for 30 minutes at 350 degrees. Meanwhile fry 4 slices of bacon until crisp, remove and allow to drain on a paper towel covered plate. After the bacon is cool, slice or crumble it into small pieces. Grate 1 cup of cheddar cheese and cut green onions or chives into slices.
Sprinkle the casserole with cheddar cheese and return to oven for 1-2 minutes until cheese is melted. Remove from oven and garnish with green onion and bacon prior to serving.