In a bowl, whisk together flour and salt, then cut in cold butter until pea-sized. Gradually add ice-cold water until dough just comes together. Chill for 15 minutes.

On a lightly floured surface, roll dough into a 1/8-inch-thick circle. Use a 2.5-inch cutter to stamp rounds, then gently press them into mini tart pans.

Preheat oven to 375°F. Dock each shell with a fork, freeze for 5 minutes, then bake until edges are just set and lightly golden, about 8–10 minutes.

Heat olive oil or butter in a skillet over medium heat. Add minced shallots and cook until soft and translucent, about 2 minutes.

Stir in chopped mushrooms and garlic. Cook until mushrooms release their moisture and turn golden brown, 5 to 7 minutes.

Sprinkle in fresh thyme, salt, and pepper, stirring gently over low heat for another minute until fragrant.

Spoon the warm mushroom mixture evenly into each pre-baked tart shell, pressing gently and leveling with the back of a spoon.

Return the filled tarts to the oven and bake at 375°F until filling is set and crust edges turn golden, about 10 minutes.

Let the tarts rest in their pans for about 5 minutes to firm up, then gently remove and transfer to your serving platter.