Preheat the oven to 350 degrees F. Spray a 12-count muffin tin with nonstick cooking spray.
Prepare the pie crust mix according to package instructions. Evenly divide the dough into 10 pieces. Divide the dough among the cups of the muffin tin, and press into the bottom and up the sides in an even layer to form a mini crust. Refrigerate while preparing the filling.
Heat the canola oil in a large skillet over medium heat. Add the mushrooms, garlic, thyme and onions. Season with salt and pepper. Saute until the mushrooms are golden. Set aside 10 mushroom slices to garnish the top of the tarts. Set aside 1/3 cup mushrooms and onions for Round 2 Recipe Braised Beef and Mushroom Soup.
Add the remaining sauteed mushrooms and onions with the walnuts into the bowl of a food processor fitted with a blade attachment. Pulse to coarsely chop. Add the breadcrumbs and beef broth, and a pulse a few times to mix in.
Spoon the mushroom and walnut mixture into the pie crusts in the muffin tin. Top each with a reserved mushroom slice. Bake until the crust is golden, 20 to 25 minutes. Remove from the oven and cool in the tin about 30 minutes before carefully removing the tarts.