The Preparation

1. Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish.
2. In a large skillet, cook the Italian sausage over medium-high heat, breaking it up with a spoon, until it’s fully browned and cooked through. Drain any excessive fat, leaving about 1-2 tablespoons in the pan.
3. Add the chopped yellow onion to the skillet with the sausage fat (or a tiny drizzle of olive oil if needed) and sauté until softened and translucent, about 5-7 minutes. Stir in the chopped scallions and fresh rosemary and cook for another minute until fragrant. Remove from heat and set aside.
4. In a large mixing bowl, whisk together the eggs, milk, Greek yogurt (or heavy cream), Dijon mustard, and cayenne pepper until well combined. Season generously with salt and freshly ground black pepper.
5. Spread half of the cubed baguette pieces evenly in the prepared baking dish. Top with half of the cooked sausage and onion mixture, then sprinkle with half of the shredded Swiss cheese, Parmigiano-Reggiano, and crumbled feta.
6. Repeat the layers: add the remaining baguette cubes, then the rest of the sausage and onion mixture, and finally the remaining cheeses.
7. Pour the egg mixture evenly over the entire strata, making sure all the bread cubes are thoroughly moistened. Press down gently with a spatula to help the bread soak up the liquid.
8. Cover the baking dish loosely with plastic wrap and refrigerate for at least 4 hours, or ideally overnight, allowing the flavors to meld and the bread to fully absorb the egg mixture.
9. Remove the strata from the refrigerator about 30 minutes before baking to come closer to room temperature. Bake, uncovered, for 50-55 minutes, or until the strata is golden brown, puffed, and a knife inserted into the center comes out clean.
10. Let the strata rest for 10-15 minutes before serving to allow it to set, then slice and enjoy.