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.
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.