Pre-heat your oven to 375F.
Heat a pan over medium low heat. Add the butter and onions or shallots and cook until they start to caramelize.
Add thyme leaves, season with salt and pepper and cook one minute more.
Peel and slice potatoes, 1/8 inch thick. Try to pick potatoes that are the same diameter as your muffin tins.
Coat the muffin tins with non-stick spray.
Layer each tin with two layers of potatoes. Season with salt and pepper.
Add another layer of potatoes, then spoon 1/6 of the onion mixture in each muffin tin.
Continue adding potatoes and seasoning until you reach are even with the top of the muffin tins.
Add one Tbsp chicken stock or water to each tin.
Top each galette with 1 Tbsp butter. Bake, loosely covered with foil for 20 minutes.
Remove foil and bake an additional 30 minutes or until potatoes are tender.
To serve, run something thin and flat down the side of the tin and push the potato stack up, stabilizing it with your other hand (put your hand on the top of the galette). This is a little tricky so take your time.