Place flour and salt in a food processor; pulse to combine. Add butter and pulse until mixture resembles coarse crumbs. Add ice water, 1 tablespoon at a time, until dough comes together. Form into a disc, wrap in plastic, and refrigerate for at least 1 hour before rolling out.

Heat 2 tablespoons olive oil in a large frying pan over medium heat. Add mushrooms, shallots, salt, and pepper. Sauté for 10-15 minutes.

Stir in 2 tablespoons Worcestershire sauce and cook for an additional 2-3 minutes. Remove from heat and cool.

Preheat oven to 375°F (190°C). Roll dough to ⅛-inch thickness and cut into 2.5-inch rounds. Press small circles into mini muffin tin and chill for 10 minutes.

In a large bowl, whisk together egg, milk, mozzarella, Parmesan, and thyme. Fold in cooled mushroom mixture.

Spoon 1 tablespoons of the mushroom mixture of filling into each mini tart shell. Bake for 12-15 minutes, until filling is set and lightly browned. Cool before serving.

If you prefer to make one large tart, use a 9-inch tart pan instead of the mini muffin tin.

Blind-bake the pie crust at 400°F (200°C) for 10 minutes with parchment paper and pie weights, then remove the weights and bake for another 5 minutes before adding the mushroom filling. Reduce the oven temperature to 375°F (190°C) and bake the tart for 30–45 minutes until the filling is set and golden.