For the filling:
2 tablespoons butter
1/2 pint roughly chopped button mushrooms (about 1 1/4 cups)
1/2 medium onion, finely chopped (about 2/3 cup)
1 clove garlic, minced
1/2 cup thawed and drained frozen chopped spinach
1/2 teaspoon kosher salt
1/2 teaspoon freshly cracked black pepper
1/4 teaspoon cayenne pepper
1/8 teaspoon fresh grated nutmeg
1 tablespoon all-purpose flour
3/4 cup milk, 2% or higher
1 cup freshly grated Parmesan cheese, divided
1 ounce cream cheese, at room temperature
1/4 cup diced fresh mozzarella
1/2 cup lump crab meat
1 teaspoon smoked paprika
For the crust:
1 1/2 cup all-purpose flour
2 teaspoons kosher salt
1 teaspoon sugar
1/2 cup (1 stick) unsalted butter, cubed and chilled
1/2 teaspoon seafood seasoning (I used Old Bay)
3-5 tablespoons ice cold water
In a medium skillet over medium heat add the butter and heat until it foams. Add the mushrooms and onions and cook until they are tender, about 5 minutes. Add the garlic and spinach and cook until the garlic is fragrant, about 2 minutes.
Season the mixture with salt, pepper, cayenne, and nutmeg then add the flour and cook, stirring constantly, for 3 minutes to cook out the raw flour taste. Add the milk and bring to a simmer, stirring frequently, until the mixture thickens, about 5 minutes.
Once thickened reduce the heat to medium low and add 3/4 cup of the Parmesan, cream cheese, and mozzarella. Stir until all the cheeses are thoroughly combined then add the crab and mix well. Remove the pan from the heat and allow to cool while you prepare the crust.
Heat the oven to 350 F.
In a large bowl sift together the flour, sugar, and salt. Add the butter and, using your fingers, rub it into the flour until the mixture looks like coarse sand studded with pea sized pieces of butter. Add the seafood seasoning and gently mix to combine.
Add 3 tablespoons of water and mix until the dough forms a rough ball. Add more water, one tablespoon at a time, if needed, until the dough forms a smooth ball. Turn the dough out onto a well floured surface and gently press the dough into a rectangle, then fold the dough in half and press it out again. Repeat this folding process two more times. Wrap the dough in plastic wrap and refrigerate for 15 minutes.
Once chilled roll out the dough to 1/8-inch thick, about a 12-inch circle, turning the dough often to make sure it does not stick. Dust the surface with additional flour if needed. With a medium biscuit cutter (about 2 1/2-inches) cut out 24 rounds. You may need to re-roll the scraps to get to 24, but you can only do this once otherwise the dough may become tough. Press the dough circles into the cups of a mini-muffin pan and chill for 10 minutes.
Bake the dough cups for 10 minutes, or until the dough is set but not golden, then remove the pan from the oven and divide the filling evenly among the cups. Top the tarts with the reserved Parmesan and a dusting of smoked paprika. Bake for 12 to 15 minutes more, or until the tarts are bubbling and the crusts become golden brown around the edges. Cool the tarts in the pan for 10 minutes before serving.