Preheat oven to 350 degrees, and grease a 9 x 13 baking pan, or a 10 x 14 oval baking dish, with olive oil.
Heat a large frying pan over medium heat, and add olive oil and onions. Saute for 1-2 minutes, or until soft. Add garlic, oregano, and artichoke hearts, and stir for another 1 -2 minutes. Add spinach and stir until wilted. Remove veggies from heat. and set aside.
Combine eggs, milk, salt, and pepper in a large mixing bowl, and whisk until well blended. Stir in feta cheese and set aside.
Add cubed bread to veggies and pour mixture in to greased pan, then pour egg and cheese mixture on top, and bake for 40 minutes.
While the strata is baking, make the salsa by combining the tomatoes, olives, oregano, and olive oil in a bowl. You can also make this ahead of time and keep in the refrigerator until ready to use.
To serve, portion out a piece of strata, and place salsa on top.