Combine the 1/2 cup of butter, cream cheese, and 1 1/4 cups of flour in a mixing bowl and beat to make a soft dough. Shape into a disk and wrap in plastic wrap; refrigerate for 1 hour.

Put the mushrooms and shallot in a food processor and process until finely chopped; remove to a bowl.

Heat 2 tablespoons of butter in a skillet over medium heat. Add the mushroom mixture and cook until the liquids have evaporated. Stir the flour into the mixture and cook for 2 minutes, stirring constantly. Remove the mushroom mixture from the heat and stir in the sour cream, salt, pepper, and thyme. Heat the oven to 375 F.

Remove the dough from the fridge and shape it into 1-inch balls. They will each weigh about 1/2-ounce or 14 grams. Place the dough balls in the mini muffin tin and make a deep indentation in each one with your thumb. Push the dough up the sides of each muffin cup.

Fill each lined muffin cup with a rounded teaspoon of the mushroom duxelles mixture and top with a cube of fontina cheese.

Bake the tartlets for 15 to 20 minutes or until the crust edges are browned and the filling is puffy (The filling will deflate when cooled).

Remove the muffin pan to a rack to cool slightly. Use a narrow spatula or knife to help remove them. Serve warm or at room temperature.