Prepare a 9x13 inch baking pan by spraying it with cooking spray. Preheat oven to 400 degrees F.
Prepare sauce by melting butter in a large saucepan on medium heat. Add flour and whisk continuously as it cooks, until golden. Gradually add milk while whisking. Heat may be turned up slightly as you cook and whisk, until the mixture thickens, which takes about ten minutes. Take care to not over heat or the sauce will scald.
Add garlic, seasoning salt, salt, pepper and thyme to the mixture, stir in well. Add 2 cups of the Monterey jack cheese to the sauce and stir until melted.
Lay half of the potato slices onto the bottom of the baking pan. Sprinkle about half of the minced onion evenly over the potatoes. Pour half the sauce over potatoes. Layer remaining potatoes over sauce, sprinkle on remaining minced onions and 1/2 cup of Monterey Jack cheese. Pour remaining sauce over the top as evenly as possible, trying to make sure that all potatoes are covered by the sauce.
Cover tightly with foil and bake for one hour.
Remove from the oven, remove foil and sprinkle remaining cheese over the top. Place back into the oven, uncovered and bake for another 15 minutes or until cheese is melted. If desired, broil for 1-2 minutes for a golden top. As always, when broiling watch closely as dishes can go from browned to burnt quickly.
Allow to cool for at least 10-15 minutes prior to serving.