In a small pot over low heat melt butter.  Add flour and whisk smooth and cook for about
2 minutes.  Add milk and cook until sauce
has thickened, about 3-5 minutes.

Stir in garlic powder and grated cheese.  Remove from heat.

Brush a 8 x 8 baking dish with butter.  Place half of the potatoes in one layer and
then top with half of onion slices, thyme leaves, salt and black pepper.  Top with half of cheese sauce.

Layer remaining half of potatoes and onion, thyme leaves,
salt and black pepper then top with the rest of the cheese sauce.  Sprinkle grated cheese on the top.

Cover dish with foil and place into an oven preheated at 350
degrees and bake for 45 minutes covered.  
Test the potatoes to see if tender, then remove foil and bake for an
additional 15 minutes or until golden brown.