Preheat oven to 180 C / Gas 4. Line a rimmed baking tray with baking paper.
Make a V-shaped cut in the baguette, leaving ends intact. Lift out V-shaped wedge. Hollow out baguette gently to make a long, skinny bread bowl, being careful not to cut through the bottom or sides. Place baguette on the prepared baking tray.
Whisk eggs in a bowl. Add milk; whisk until well blended. Stir in cheese, spinach, bacon, sun-dried tomatoes, black pepper and chilli flakes. Pour mixture slowly into the hollowed-out baguette, spreading evenly with a spoon.
Bake in the preheated oven until egg mixture is completely set and cheese is lightly browned, about 30 minutes. Cool for 5 minutes; cut into thick slices.