Place the spinach on a clean kitchen towel and squeeze firmly over the sink to remove as much liquid as possible. Set aside.

Heat the oil in a large skillet over medium-high heat. Add the onion and cook, stirring occasionally, until translucent, about 5 minutes. Add the garlic; cook, stirring often, until fragrant, about 30 seconds.

Stir in the spinach; cook, stirring often and breaking up with a wooden spoon, until it looks dried out, about 2 minutes. Set aside to cool slightly, about 5 minutes.

Reserve 1/4 cup of the spinach mixture. Transfer the remaining spinach mixture to a medium bowl and add mascarpone cheese, 1/2 cup of Parmesan, 1/2 teaspoon salt, and 1/2 teaspoon black pepper. Stir until well combined and set aside.

Whisk the eggs, half-and-half, crushed red pepper, reserved 1/4 cup spinach mixture, 1/4 cup Parmesan, remaining 1 1/2 teaspoons salt, and 1/2 teaspoon black pepper in a large bowl until well combined.

Lightly grease a 13x9-inch baking dish. Arrange the bottom halves of the English muffins in the dish in three rows of two.

Evenly spread the spinach-mascarpone mixture over the muffins. Cover with the top English muffin halves.

Pour the egg mixture over the casserole, then press down on the English muffins to submerge. Sprinkle with remaining 1/4 cup Parmesan.

Cover the dish with foil and refrigerate for at least 1 hour or up to 12 hours.

Preheat the oven to 350°F. Uncover and bake the casserole in the preheated oven until just set and the tops of the English muffins are golden brown, about 45 minutes.

Let the casserole cool 10 minutes before serving.