For the hot milk pastry. Cut the lard into small chunks. In a medium saucepan, over medium heat, combine the milk and the lard, heating until it is almost boiling. Combine the salt and the flour, then slowly add the hot milk to the flour mixture, stirring with a wooden spoon until a dough forms. Or if you have a food processor, you can combine the flour mixture and hot milk, pulsing until a dough forms into a ball.
Flour your counter. Form the dough into a disc shape and let it rest for about 20 minutes, or until cool. Once the pastry is cool, you can wrap it and refrigerate it up to two days, but you have to let it come to room temperature first.
Place the cast iron skillet into your grill and light all of the burners. The one or two under the skillet should be set to medium low, and the ones under plain cooking grids set to high.
In a bowl, combine the flour, salt, pepper, and thyme. Add the beef cubes, tossing them until coated in flour. Thread them on to skewers and set aside.
In a cast iron dutch oven, or oven safe pot, over medium heat melt 1 tbsp. of butter and add the mushrooms, allowing them to sweat and cook. While the mushrooms are cooking, add the butter to the cast iron skillet on the grill and cook the onions down slowly, allowing them to caramelize. Sear the beef skewers over direct heat on the cooking grids with the burners set to high.
Roll the pastry dough out to about 1/4 –inch thick on a floured surface. Brush a muffin tin thoroughly with melted butter, then, using a round cookie cutter to make the initial shape, press the dough-round into the muffin tin. Fill the dough with the cooled stout stew, then cut another round to make the top of the mini pie. Roll the two edges of the pastry together. Cut a small vent in the top. Repeat until all of the dough and stew have been used up.
Chill the meat pies in the fridge while you prep the grill. Clean the cast iron and set atop the cooking grids on the grill so that there is a burner to each side for an indirect cooking set up. For added heat management and even better results, place a pizza stone atop the cast iron. Preheat the grill to 375°F by lighting the two outside burners that are on either side of the cast iron setup.
Place your muffin tin filled with chilled pies onto the preheated baking stone, which is sitting on the cast iron. Bake the meat pies for 30 minutes or until the pastry is golden brown.