Preheat oven to 450°. Line a baking sheet with parchment paper.
In a large sauté pan, heat oil over medium-high heat. Add mushrooms; cook, stirring occasionally, until golden brown and softened, about 10 minutes.
Add garlic, ¾ teaspoon salt, pepper, thyme, and chives; cook until aromatic, about 1 minute more. Drain any excess moisture.
In a small bowl, combine ricotta, ½ cup Gruyère, and remaining ¼ teaspoon salt.
Unroll piecrust. Spread 1 cup cheese mixture onto crust in a thin layer, leaving a 1½-inch border. Top with mushroom mixture and remaining ½ cup Gruyère; gently fold edges of dough over filling.
Brush dough with egg; bake until crust is golden brown and cheese has melted, about 25 minutes. Let rest for at least 5 minutes before slicing. Garnish with chives and thyme, if desired.
Notes
*Baby portobello mushrooms are often sold as creminis or “Baby Bellas.”
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/mushroom-galette/