Grease one 9 x 13 inch baking dish.  Cube the bread. Toss the cubes with 2 tablespoons of olive oil, a pinch of salt and a few grinds of pepper.  Heat a large skillet over medium-high heat, cook the bread cubes in the skillet until toasted.  This will take about 8 minutes, stir frequently.  As soon as they are golden, remove and place in a a large  bowl.  In the same skillet add 2 tablespoons of olive oil and heat skillet over medium-high heat.  Add the mushrooms in one layer and let cook until they start to brown, about 3 minutes.  Do not turn or touch the mushrooms for the first 3 minutes - you want them to get crusty brown.  After 3 minutes stir and cook an additional 2 minutes.  Add the garlic, thyme and a dash of salt and pepper.  Cook 2 minutes, then add the spinach.  Continue cooking until the spinach is wilted.  Remove from the heat.  

In the prepared pan layer 1/2 of the bread cubes, 1/2 of the Gruyere cheese, 1/2 of the Parmesan cheese and 1/2 of the vegetable mixture.  Repeat.  Combine the eggs and half and half - add 1/2 teaspoon salt and a few grinds of black pepper.  Pour over the ingredients in the pan.  Cover and refrigerate overnight.  

In the morning, preheat the oven to 350 degrees.  Bring the casserole out of the refrigerator while the oven is heating.  Bake the casserole uncovered for 50 - 55 minutes or until puffed and golden brown.  Cool for 15 - 20 minutes prior to serving.  Serve warm or at room temperature.