Peel the onions and slice off just enough of the ends to get flat bottoms and tops on the onions. Cut the onions in half around the equator and use a paring knife to gently pry some of the center of each half onion. I like to have 3 or 4 rings left on the outside. By the way, if you happen to go too deep and cut all the way through the bottom of the onion, just shove a piece of the cut out onion in the bottom. Save the extra pieces of onion for another time. Place the onion halves, cut sides up, in a casserole dish that has been sprayed with cooking spray.
In a mixing bowl, tear up the toast and add in the cheese and herbs. Add in 1 Tablespoon of vegetable broth and combine. Place a portion of this filling in each of the onion halves. Pour the remaining broth over the onions and then into the bottom of the casserole dish. Top each onion half with about a half teaspoon of butter and throw the fresh thyme sprigs on top. Spray the tops of the onions with cooking spray. Cover the dish with foil and bake for 30 minutes. Remove the foil, baste the onions with the pan juices and cook for another 20 minutes. Remove from the oven and serve.