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, 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 onion and garlic and cook to soften, 5-6 minutes. 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.
Bring the strata to room temperature for about 30 minutes before you bake. Pre-heat the oven to 350°F with the rack in the center of the oven.
Set the casserole on a baking sheet and bake uncovered for 1 hour. Let stand 10 minutes and serve.