Heat oil in a Dutch oven over medium heat. Add garlic and spinach; cook 3 minutes or until spinach wilts and liquid almost evaporates. Cool.
Spread 1 can of tomatoes over the bottom of a 13 x 9-inch baking dish coated with cooking spray. Arrange half of the bread slices over tomatoes; top with 1 can of tomatoes, spinach mixture, and remaining bread slices. Combine milk, salt, pepper, and eggs in a large bowl; stirring with a whisk. Pour over bread slices; sprinkle with cheese. Cover and refrigerate 8 hours.
Preheat oven to 375°.
Uncover strata. Bake at 375° for 50 minutes or until set.