Caramelize onions with thyme until brown but not completely finished
Layer potatoes as seen, with onions, and slices of butter between each layer
Cut a few rings of potato for the final layer, saving the scraps for pomme puree
Fill with stock until almost completely covered
Bake at 375f for 70 minutes
Test doneness with a fork
If too liquidy you can drain some of the stock with a baster then cook 10 more minutes to crisp up
