In a bowl, mix together the beef rub and granulated garlic.
Brush both sides of the steak with vegetable oil.
Apply a medium coat of beef-garlic rub to both sides of the steak. Rest the steak in the fridge while you set the smoker.
Preheat the grill to 275 degrees F (135 degrees C) with a direct hot zone and an indirect cold zone. When hot, add a chunk of pecan wood.
Place the steak onto the preheated grill in the indirect zone away from the fire.
Smoke the steaks until the internal temperature is about 110 degrees F (43 degrees C), approximately an hour. While smoking prpeare the sauce and potatoes
Wine Butter Sauce
Heat the red wine in a saucepan over high heat to a boil. Continue to boil until reduced to about 2 tablespoons of syrupy red liquid. Lower the heat.
Cut the chilled butter into small pieces. Slowly stir into the wine syrup one piece at a time until the sauce becomes silky and thick enough to coat the back of a spoon. Season to taste with salt and pepper.
Remove from the heat and keep warm while you grill the steak.
Mushroom Mashed Potatoes
Sauté the minced white mushrooms with the butter and garlic over medium heat until the mushrooms release their juices, about 10 minutes.
Add a bit of chicken stock to cook the mushrooms to reduce them into a wet paste-like consistency. This will be about 1/4 cup yield.
Mix the sautéed mushrooms with two cups of cooked mashed potatoes.
Add chicken stock, a tablespoon at a time, to get the mashed potatoes to the desired consistency.
Taste then add the salt and white pepper as needed. Keep warm and set aside.
Steak - Grilling
Move the steaks from the indirect zone to the direct zone to get a char.
Brush on the melted butter using a few rosemary sprigs tied together as a brush. Remove when medium rare, about 128 degrees F (53 degrees C) or the doneness you like.
Rest for 15 minutes.
Brush the steak with some melted butter and a light sprinkle of beef rub as a finishing rub before slicing the steak diagonally against the grain.
Serve the flat iron steak slices on some of the mushroom mashed potatoes.
Drizzle the red wine sauce on the steak or serve on the side. Garnish with chopped parsley.