Caramelized Onion, Whole Grain Mustard, and Gruyere Grilled Cheese

For the caramelized onions (will make enough for 3 grilled cheeses):

large white or yellow onions, halved and sliced into strips

plus 1 tablespoon olive oil

water

salt

For the grilled cheese:

slices fresh sourdough bread

mayonnaise

gruyere cheese, grated

Maille Old Style Whole Grain Dijon Mustard

caramelized onions, from above

few sprigs of fresh thyme