Instructions

• Thinly slice the fennel bulb and the 1/2 red onion; set the slices aside.
• Heat a large nonstick skillet over medium heat. Add the 1 pound pork breakfast sausage and cook, breaking it up with a wooden spoon or spatula, until no longer pink, about 5–7 minutes.
• Use a slotted spoon to transfer the cooked sausage to a paper towel–lined plate to drain, leaving the pan grease in the skillet.
• Add the sliced fennel and sliced onion to the greased skillet with 1/2 teaspoon of the kosher salt. Cook, stirring every couple of minutes, until the vegetables are translucent and starting to brown, about 15–20 minutes.
• While the fennel and onion cook, crack the 12 eggs into a large bowl. Add the 1/4 cup milk, the remaining 1/2 teaspoon kosher salt, and the 1/4 teaspoon ground black pepper. Whisk until fully combined. Grease a 9x13-inch baking dish and set it aside.
• When the fennel and onion are done, add the 5 ounces fresh baby spinach to the skillet and cook until wilted, about 3–4 minutes.
• Spread the drained cooked sausage in an even layer in the prepared baking dish. Spread the cooked vegetables evenly over the sausage.
• Pour the egg mixture evenly over the sausage and vegetables, making sure most of the vegetables are covered by the egg.
• Cover the baking dish with plastic wrap and refrigerate overnight. (If you prefer to bake immediately, skip step 10 and proceed to step 11.)
• In the morning, remove the covered dish from the refrigerator and let it sit at room temperature for about 30 minutes.
• Preheat the oven to 350°F. Bake the casserole for 35–40 minutes, until set in the center (a toothpick inserted in the middle should come out mostly clean). Let the casserole rest for 5 minutes before slicing and serving.