Pre heat oven to 400 degrees F with the rack in the middle. Prepare a baking sheet with tin foil or a Silpat liner. Toss sweet potato slices in olive oil, garlic salt, pepper & smoked paprika. Arrange sweet potatoes on the sheet pan in a single layer. Bake about 20 minutes until potatoes are soft & slightly browned. Remove from oven & let cool a bit.
Melt butter in a sauce pan, whisk in flour. Stir in milk & bring to a simmer. Set aside.
Spray a 3 quart baking dish with cooking spray. Layer potatoes & 1 cup of the cheddar cheese, repeat this for 2nd layer. Add 3rd layer of sweet potatoes, pour milk mixture over the top. Top with 3rd cup of cheddar cheese & mozzarella.
Cover casserole dish with tin foil. Bake for 60 minutes. Carefully remove the foil & broil for 5 -10 minutes. Cheese should be slightly browned & bubbling. Be sure it does not burn. Remove from oven & let sit a few minutes. Sauce will thicken as the casserole cools.
Top with avocado, green onions & bacon crumbles. Serve with salsa & sour cream.
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