Defrost the spinach in the microwave, and then wring out any excess liquid.
Heat the EVOO in a skillet over medium-high heat. Brown and crumble the spicy breakfast sausage, and then remove the crumbles to a plate. Return the skillet to the heat. Reduce the heat a little and melt the butter. To the skillet, add the onions and garlic and cook to soften, 5 to 6 minutes. Then add the spinach and season with salt, pepper and a little nutmeg. Turn off the heat and combine with the sausage bits.
Arrange half of the bread in a casserole dish.
Whisk the eggs with the milk, Dijon and some salt and pepper.
Scatter half of the sausage-spinach mixture over the bread, top with half the cheese, pour over half the eggs, repeat. Cover and store in the fridge at least overnight and up to a few days.
Bring the strata to room temperature for about 30 minutes before you bake. Preheat the oven to 350 degrees F with the rack in the center of the oven.
Set the casserole on a baking sheet and bake uncovered 1 hour. Let stand 10 minutes and serve.