Everything you love about French Onion soup, compressed between two pieces of bread: sweet caramelized onions cooked in rich beef stock, Gruyère, and fresh thyme. A two-for-one soup and a sandwich.
Melt the butter over low heat in a large sauté pan, and then add the onions, trying to spread into 1 layer. Sprinkle generously with salt and a large pinch sugar. Sauté over very low heat until very tender and caramelized, about 25 minutes, stirring often to prevent burning.
Add the beef stock and thyme, increase the heat to medium, and cook until the beef broth is almost completely absorbed, but still a touch soupy.
Preheat the broiler and line a baking sheet with aluminum foil.
Brush 1 side of each bread slice with the canola oil, and season lightly with salt. Divide the Gruyère slices among the un-oiled sides of the bread. Top 2 of the slices with the onion mixture. Transfer the bread to a baking sheet, and broil until the cheese is melted, about 3 minutes. Top the onion slices with the plain cheesy slices and serve.