2 tbsp. unsalted butter
1 med onion, thinly sliced
4 cups whole milk (or 2 cups milk 2 cups water)
2-3 garlic cloves, smashed
1 heaping teaspoon Dijon mustard
2 1/2lb(about 6 med)potatoes peeled
1 1/2 cups grated Cheddar cheese, divided
3/4 cup heavy cream
Salt and freshly ground black pepper
Preheat oven to 375 degrees. Lightly grease med sized (1.5-2 qt) gratin or baking dish.
In large Dutch oven melt the butter over med heat. Add onions and sauté until softened, about 5 minutes. Add milk, garlic, and Dijon mustard, bring to a gentle boil over med heat. Add generous amount of salt & pepper.
Slice the potatoes to 1/8 inch thickness, using food processor for even thickness. (do not rinse potatoes) Add potatoes to the milk and allow to simmer until potatoes are almost tender- about 10 minutes. Using a slotted spoon, transfer half of potatoes and onions to baking dish. (Discard milk or save for another culinary use) Season generously with salt and pepper and top with 3/4 cup cheese. Cover with remaining potato mixture, season again with salt & pepper and top with remaining cheese. pour cream evenly over the potatoes and cheese.
Bake until crisp and golden on top. 50 minutes to 1 hour. Allow to cool for 10 minutes before serving.