Place potatoes and 1 tbsp salt in sauce pan, add water to cover, boil until tender (12-17 minutes) drain
Add potatoes, butter, garlic, and 1 1/2 tsp salt to a food processor. Pulse until butter is melted and incorporated (10 pulses)
Add 1 cup of milk and continue to process until smooth (about 20 seconds) scraping down 1/2 way thru
Return potato mixture to saucepan and set over medium heat.
Stir in cheeses, 1 cup at a time, until incorporated, stirring vigorously (3-5 minutes)
If the mixture is difficult to stir add more milk, 2 tbsp at a time.
Season with salt and pepper to taste, serve immediately