Carefully cut potatoes into quarters. Place the potatoes in a medium-sized pot and fill with cold water until the potatoes are just covered with water. Place potatoes over medium-high heat and bring to a boil. Boil for about 10 minutes or until potatoes are fork tender.
Preheat oven to 350 degrees. Drain the potatoes in a colander and return to the cooking pot. Add butter; add milk in increments of 1 tablespoon until just moist. Using a potato masher or a fork, mash potatoes until there are no lumps, adding milk as needed to make them fluffy and creamy. Season with salt and pepper. Add 1/4 cup of the sharp cheddar cheese, continue to mash unti cheese has melted.
Place mashed potatoes in a small baking dish. Sprinkle remaining cheese over the potatoes. Sprinkle bacon over the cheese; top with chopped tomatoes. Place in the oven for about 10 minutes or until the cheese has melted on top. Remove from oven and sprinkle with green onions.