Two -Potato Gratin

2 each medium baking potatoes and sweet potatoes, peeled and cut into 1/4 thick slices (about 7 cups
2 quarts chicken stock
2 TBLS canola oil
3 TBLS flour
2 garlic cloves, crushed
1 1/2 cup milk
2 thyme sprigs
3.4 tsp salk, divided
pepper
5 oz shredded cheese (mixture of swiss, colby and mozzerela or whatever)
2 TBLS chopped chives, divided ( fresh is really good)
1 1/2 tsp fresh thyme
1/3 cup Parmigiano-Reggiano cheese, grated

Preheat oven to 350.  Bring potatoes to boil in large pan.  Remove from heat and carefully remove potatoes using a slotted spoon.  Reserve the cooking liquid.  Arrange potatoes on jelly roll pan to cool slightly.  Strain cooking liquid and reserve one cup.  Toss the rest.  In medium saucepan add canola oil, sprinkle four over oil; cook 1 minute stirring with whisk.  Add garlic, cook 2 more minutes, continue stiring.  Combine milk and reserved 1 cup cooking liquid.  Gradually pour milk mixture into flour mixture--keep stirring.  Add thyme sprigs and bring mixture to a boil; cook 4 minutes or until slightly thick--keep stirring. Remove from heat add salt, pepper and shredded cheeses.
Spread 1/2 cup sauce in bottom of oven safe 2 quart baking dish, coated with cooking spray.  Arrange a single, flat layer of sweet potato and then baking potato slices over sauce.  Keep  alternating potatoes in shingle-like fashion.  Sprinkle evenly with remaing salt and chives and thyme.  Pour remaining  sauce over potatoe mixture.  Bread crumbs can be added now, if you want.   Sprinkle with Parmigiano-Reggiano cheese.  Bake for 1 hour.  Remove dish from oven preheat broiler.  Place dish back in oven to broil for 3 minutes.  Add more chives if desired.  Serve warm.