Heat a heavy bottomed saucepan over medium heat. Add in onions and 1 tbsp. olive oil. Cook for 1 minute until heated through, then turn heat down to low and add in 1/4 cup water, remaining olive oil, and salt. Cook, stirring occasionally, for 1 hour.
The onions will go from white to translucent, and finally will begin to turn golden brown. If the onions are looking dry, add 1-2 additional tbsp. of water, as necessary. Once onions have reached a nice medium golden brown colour, remove from heat and allow to cool slightly. They can be stored for up to 1 week in the fridge.
Prepare the Grilled Cheese
Heat a skillet over medium heat. Spread mayonnaise over both slices of sourdough, then spread Maille® Old Style Mustard over the opposite side of one of the slices.
Place both slices, mayonnaise side down, onto skillet. Dollop the mustard on a slice of bread with caramelized onions and a sprinkle on a few leaves of fresh thyme. Carefully place grated gruyere over the other slice. Cook for 2-3 minutes, until cheese is melted completely. Press sandwich together and serve.