Place bread slices in a single layer on wire racks; cover loosely with a clean kitchen towel or paper towel and let stand for 8 to 12 hours or until dried. (Or preheat oven to 300 degrees F. Place bread slices in a 15x10x1-inch baking pan; bake for 10 to 15 minutes or until dried, turning once.) Tear slices into large pieces.
In an extra-large nonstick skillet cook spinach in hot oil over medium heat for 1 to 2 minutes or just until wilted, turning frequently with tongs. (If using kale, cook for 6 to 8 minutes or just until tender.) Coarsely chop spinach or kale.
Lightly grease a 2-quart rectangular baking dish. Arrange half of the bread pieces in the prepared baking dish. Add spinach and Canadian bacon. Top with the remaining bread pieces.
In a medium bowl beat together eggs, egg whites, 1/4 cup sour cream, flour, dry mustard, lemon peel, and pepper. Stir in 1 2/3 cups milk until combined. Pour evenly over the layers in dish. Using the back of a large spoon, gently press down on layers. Cover and chill for 2 to 24 hours.
Preheat oven to 325 degrees F. Bake strata, uncovered, for 55 to 60 minutes or until a knife inserted near the center comes out clean. Let stand for 10 minutes before serving.
Meanwhile, for sauce, in a small bowl stir together 1/3 cup sour cream and Dijon mustard. Stir in enough of the 1 to 2 tablespoons milk to reach drizzling consistency. Drizzle sauce over strata. If desired, garnish with thyme.