1. Preheat the oven to 400°.
2. Blind-bake the piecrust first: Press the pastry evenly into your 9-inch pan and prick the bottom of the pastry all over with a fork.
3. Line the pastry with parchment paper and fill the pan with pie weights or dried beans. Bake until the pastry begins to turn slightly golden around the edges, 15–20 minutes. Remove the shell from the oven and set it aside to cool.
4. While your crust is cooling, melt 2 tablespoons of the butter in a large skillet over medium-high heat.
5. Add the mushrooms and garlic, and sauté them until the mushrooms are golden brown, about 8 minutes. Remove the mushrooms from pan and reserve them for later.
6. Add the remaining butter, the leek, and the rosemary to the skillet, and sauté until the leek softens, about 5 minutes. Pour in the cream and cook for 2 minutes before stirring in the mushrooms. Remove the pan from the heat, and season everything with salt and pepper.
7. Spread the mustard around the bottom of your cooled piecrust. Sprinkle half of the cheese over the mustard, then spoon in the mushroom/leek mixture. Sprinkle the rest of the cheese on top and place the tart in the oven.
8. Bake until edges of the crust are golden brown and the cheese is bubbly, about 15 minutes. Let the tart rest 15 minutes before cutting and serving.