Heat the olive oil in a large sauté pan over medium heat. Add the onion and season with salt and pepper and cook, stirring occasionally, until caramelized, about 15 minutes. Transfer to a bowl.

In the same pan over medium heat, add an extra 1/2 tablespoon of the olive oil. Add half of the spinach, season with salt and pepper and cook, stirring, until wilted, 2 to 3 minutes. Transfer to a colander. Repeat with another 1/2 tablespoon of olive oil and the remaining spinach. Using a rubber spatula, press out the excess moisture once it’s all cooked.

In a large bowl, toss together the toasted bread, caramelized onion, spinach, artichoke and cheeses. Transfer to a Pyrex Deep baking dish (7x11). In a bowl, beat together the eggs, heavy cream, the 1 teaspoon of salt and ¾ teaspoon of black pepper. Pour over the bread mixture. Cover with aluminum foil and refrigerate for at least 2 hours or up to 12 hours.

Preheat an oven to 350°F.

Transfer the Pyrex, foil and all, into the oven and bake for 20 minutes. After 20 minutes, remove the foil and continue baking until the top is browned and the strata is cooked through, 40 to 50 minutes more.