1. Preheat oven to 350 degrees Fahrenheit.
2. Oil or butter a 12 Inch Skillet and arrange potatoes in a thin layer on the bottom. Set aside half the garlic and rosemary. With the remaining half, sprinkle a handful of chopped garlic and rosemary on the potato layer, and a pinch of salt. Add another layer of potatoes on top, and repeat with garlic, rosemary and salt. Add another layer of potatoes and repeat if necessary, until potatoes reach about an ¾ inch below the rim of the skillet. 
3. On low heat in a medium saucepan, sauté the remaining garlic in butter for 4-5 minutes, stirring to prevent browning. Add the rest of the chopped rosemary, reserving just pinch for the garnish, and stir for an additional minute. Add 1 tablespoon flour and stir constantly for 1-2 minutes, until flour begins to smell slightly nutty. Add milk and cream and increase heat to medium, stirring for 3-4 minutes, until liquid has reduced slightly. Add mustard, 1 teaspoon salt, and 2 cups sharp cheddar cheese. Stir until all the cheese has melted and remove from heat.
4. Pour milk/cream mixture over the potatoes, filling the skillet to just above the potatoes, and bake until surface begins to brown, about 30-45 minutes for a mini skillet and up to 90 minutes for a 12 Inch Skillet. Using pot holders, remove skillet from the oven and sprinkle the remaining half cup of cheese on top. Place back in the oven for an additional 5 minutes. Garnish with any remaining rosemary and serve while hot.

Kitchen Note: Fits one 12 Inch Skillet or one 10.25 Inch Skillet, plus two 6-inch mini skillets.