Preheat oven to 350 degrees F (175 degrees C). Grease a 9-inch round cake pan with cooking spray.
Cook and stir bacon in a large skillet over medium heat until some of the fat is released, about 2 minutes. Add onion; cook and stir until onion turns golden and bacon is crispy, 3 to 5 minutes. Stir in spinach and cook until wilted, about 2 minutes.
Whisk milk and eggs together in a bowl. Soak croissant cubes in the milk mixture, 1 to 2 minutes. Stir in bacon mixture and drippings from the skillet. Fold in Monterey Jack cheese. Pour mixture into the prepared pan.
Bake in the preheated oven until golden brown, about 30 minutes. Let stand for 10 minutes before slicing.