Peel and clean the potatoes. Cut the potatoes in half. Put them in a medium pot with cold water, and make sure the potatoes are covered by water completely. Season with salt generously. Bring to a boil over high heat. Once the water starts to boil, simmer and cook until the potatoes are fork tender, about 25 – 35 minutes.
Preheat the oven to 450˚F (230˚C).
Drain the potatoes, and return them to the pot. Mashed the potatoes with a masher or big fork until there is no lump.
Heat the milk in the microwave for 40 seconds or until warm.
Put the butter into the mashed potatoes. Mix together with a spatula. Stir in the garlic. Add 2/3 cup warm milk and stir together. If needed, add more warm milk to reach a consistency that you prefer, but keep in mind that sausage and corn will loosen the mashed potatoes a bit more. Season with salt and pepper.
In a medium pan over medium heat, add the sausage. Cook until nicely brown, about 2 – 3 minutes. Add the corn and cook until they start to get colors. Transfer the mixture to the mashed potatoes. Mix everything well.
Transfer the mashed potatoes to a medium ovenproof casserole or baking dish. Top with cheeses. Bake in the oven until golden brown, about 20 minutes. Cool for 10 minutes before serving.