Preheat oven to 425°F with a rack in the center. Place a large ovenproof skillet (preferably cast-iron) on the center oven rack to preheat until step 3.

Transfer mascarpone to a liquid measuring cup; add enough water to make ¾ cup total; whisk to combine. In a large bowl, combine mascarpone mixture, 3 large eggs, ½ cup flour, 1 teaspoon salt, and a few grinds of pepper; whisk until batter is very smooth and a few bubbles are visible on the surface, about 1 minute. Finely grate all of the Parmesan.

Carefully remove skillet from oven, then add 3 tablespoons butter, swirling to melt and making sure butter coats the bottom and sides of skillet. Pour batter into skillet and top with half of the Parmesan. Bake on center oven rack until Dutch baby is puffed and browned in spots, 15–27 minutes.

Trim stems from mushrooms, then thinly slice caps. Finely chop 2 teaspoons garlic. Chop parsley leaves and stems. In a small bowl, stir to combine all of the ricotta, 2 teaspoons oil, and 1 teaspoon of the chopped garlic; season to taste with salt and pepper and set aside until ready to serve.

Heat 1½ tablespoons oil in a second large skillet over medium-high. Add mushrooms and cook, stirring occasionally, until tender and browned, about 5 minutes. Season with salt and pepper. Add spinach, half of the parsley, remaining chopped garlic, and 2 tablespoons butter. Cook until spinach is wilted, 1–2 minutes. Season to taste with salt and pepper.

Spoon seasoned ricotta over warm Dutch baby, then top with mushrooms and spinach. Garnish with remaining parsley and Parmesan. Enjoy!