Step 1

In a large heavy bottom skillet, or cast iron pan, heat oil over medium heat. Add onion and saute for 5 minutes, stirring to coat. Turn heat down to medium low. Stir occasionally and continue sauteing for 20-30 minutes. Onions will sweat out their moisture, and eventually caramelize into a beautiful deep golden brown. Add a generous pinch of salt, a splash of port, sherry, or stock (optional) and let the liquid cook off. Remove from pan and wipe it out with a paper towel, to use again.  Preheat oven to 350 F

Step 2

Spread the bread with whole grain mustard (optional). On two slices, divide and layer shredded Gruyere, top with caramelized onions, sprinkle with fresh thyme leaves, then add the rest of the cheese. “Put the lid on” the sandwiches with the other slices of bread and gently press together

Step 3

Heat 2 tablesoons butter (or a combination of butter and oil) in the same skillet over medium heat. Place sandwiches in the pan and toast, carefully flipping when perfectly golden brown and cheese begins to melt. Place in a warm oven to fully melt the cheese -which will preventing the bread from getting too dark. Cut in half and enjoy.