Preheat oven to 300 degrees. Lightly coat a large baking dish with cooking spray. I use 9x13, however you could go slightly smaller. Onions halves need to lay in a single layer.
Lay onions halves into the baking dish. Pour beef consomme over top of onions. Add water to bottom of the dish.
Place onions in the oven. Bake for 1 hour, basting every 15 minutes with beefy broth.
Onions should be translucent and soft. Top each onion halve with 1-2 slices Swiss cheese. Sprinkle 1/2 teaspoon sage on each slice. Season with fine sea salt and freshly ground pepper. Return to the oven for 3-4 minutes, or until cheese starts to melt.