1. Spray a 13 x 9-inch baking dish with nonstick cooking spray. Heat oil in a large saute pan over medium-high heat. Add artichoke hearts; cook 3 to 5 minutes or until thawed and browned. Stir in spinach; cook until wilted, about 1 to 2 minutes. Remove to a mesh strainer and gently press to release some liquid from spinach. Set vegetables aside.
2. In a large bowl, whisk eggs. Whisk in milk, salt and pepper until combined. Stir in feta, olives and cooked vegetables. Gently stir in bread cubes, tossing to coat evenly in egg mixture. Transfer to baking dish, cover with aluminum foil and refrigerate overnight.
3. Remove strata from refrigerator 30 minutes before baking. Heat oven to 350 degrees. Bake, covered, for 25 minutes. Remove foil and bake another 35 minutes. Let rest 10 minutes before serving.