Ingredients

• 1 (9-inch) pie dough round
• 2 tablespoons extra-virgin olive oil
• ½ pound small portobello mushrooms and stems, halved and then sliced crosswise ¼ inch thick
• 1 pound leeks, trimmed of dark green parts, washed thoroughly, halved lengthwise, sliced thin
• ½ teaspoon table salt
• 2 garlic cloves, minced
• 1 teaspoon minced fresh rosemary or thyme
• 4 ounces Gruyère cheese, shredded 
• 1 large egg, beaten
• 1 tablespoon minced fresh chives

Instructions

1. Adjust oven rack to middle position and heat oven to 425 degrees. Line rimmed baking sheet with parchment paper. Roll dough into 14-inch circle on lightly floured counter; transfer to prepared sheet.
2. Heat oil in 12-inch nonstick skillet over medium-high heat until shimmering. Add mushrooms, leeks, and salt and cook, stirring occasionally, until beginning to brown, about 10 minutes. Stir in garlic and herbs and cook until fragrant, about 30 seconds. Cool for 10 minutes.
3. Transfer cooked vegetables to prepared dough, leaving 2-inch border around edge. Sprinkle Gruyère over vegetables and fold border over filling around edge of tart, pinching pleated dough to secure. Brush dough with beaten egg. Bake until crust is deep golden brown, 18 to 25 minutes, rotating sheet halfway through baking. Sprinkle baked tart with chives. Let cool slightly before slicing and serving.