Preheat your oven to 350° Spray an 8 x 10½ x 2 inch deep oven safe baking dish with non stick spray.
In a pot, combine the cream, milk, garlic, salt and pepper.
Warm the mixture over a medium heat, stirring until you see bubbles appear around the edge of the pot.
Add the 6oz of grated white cheddar to the liquid and stir until smooth. Set aside
Slice the peeled sweet potatoes into thin circles.
Shingle a layer of sweet potato on the bottom of your prepared dish and season lightly with salt and pepper.
Add some of the grated Monterey Jack over the sweet potato slices.
Repeat this process until you have four layers of sweet potato and finishing with grated Monterey Jack on top.
Stir the milk and cream mixture one more time and slowly pour it over the potatoes.
Let the dish sit for 10 minutes, gently pressing the potatoes down with the back of a wooden spoon or spatula.
Place the baking dish onto a large baking sheet pan to catch any drips or spills and place in the oven.
Bake uncovered for about 70 minutes or until the surface is golden and bubbling.
Let the potatoes rest for about 15-20 minutes before serving. Covering them with foil will help keep them warm.