Grease a 9×13 inch baking dish with softened butter. Arrange bread cubes in an even layer in bottom of pan.
Heat a cast iron skillet over medium heat. Add sausage and break up into bite sized pieces with a wooden spoon. Once slightly browned add the diced onions, peppers, 1 tsp salt and 1/2 tsp pepper and sauté for about 5 minutes until soft and translucent. Add the garlic and spinach until just wilted, then turn off the heat.
Poor sausage mixture over the bread cube layer.
In a medium sized bowl, whisk together the eggs, milk, shredded cheese, and remaining salt and pepper. Poor over the casserole and cover with plastic wrap. Allow this to sit for at least 30 minutes or overnight.
When ready to bake, preheat over to 375 degrees and bake for 35-40 minutes or until golden brown and bubbly.
