Caramelized Onion Spinach Feta Savory French Toast Casserole

Heat 1 tablespoon of butter and oil in a large cast iron skillet over medium heat. Add the onions and let sauté for 5 minutes, stirring as required. Add the thyme and sugar and continue to cook for 20-25 minutes on medium low heat. Be sure to stir the onions every 5 minutes so they caramelize evenly. If at any point the onions are caramelizing too fast, turn down the heat to the low setting. Add the balsamic vinegar to deglaze the pan, allow the onions to soak it all in. Remove the onions to a plate.While the onions are caramelizing, in a large bowl, whisk together the eggs, milk, crumbled feta cheese, and salt and pepper to preference. Set aside.Add the remaining 1 tablespoon of butter to the skillet. Add the garlic and allow it to saute for 30 seconds or until it becomes fragrant. Add the spinach and let it cook down until just wilted, about 1-2 minutes. Remove spinach to the same plate as the onions. Toss to mix evenly.Position a rack in the center of the oven and preheat the oven to 375ºF if you are baking the casserole immediately.Generously spray a 9x13 baking dish with nonstick cooking spray. Add the diced french bread into the dish and press gently so the bread flattens a bit. Add the onion and spinach mixture on top, spreading it evenly. Using the back of a large spoon, press down the mixture down a bit so that it's not popping out over the dish. Pour the egg mixture over the prepared casserole. Sprinkle with the cheese and another small pinch of salt and pepper.Bake the casserole, uncovered, until the cheese melts, and the top gets all nice and golden, 45-50 minutes. Allow the casserole to cool for 10 minutes before slicing and serving.