Meanwhile, whisk together the milk and eggs. Season with salt and pepper.
Evenly distribute the spinach, onions, and cheese in the baking dish, lightly tossing together. Pour the egg mixture over the bread mixture. Cover and refrigerate for at least two hours, or overnight.
Preheat the oven to 350 degrees F. Cover the baking dish with foil and bake for 25 minutes on the middle rack. Remove the foil and continue cooking uncovered for 40-45 minutes until cooked through. If the top begins to brown too much, cover with foil until finished cooking.
Notes
*To toast the bread, I usually put it in the oven at 250-275 degrees F until dry (but not browned).
This is best served day-of to maintain the firm, crispy top, however, I actually still love eating from it the next day after heating it in the microwave. Store in the fridge.
Recipe by thyme & toast at https://thymeandtoast.com/spinach-and-feta-strata/