Cut the steak into 1 1/2" cubes and transfer to a large ziplock bag. Peel and cut the red onion in half. Cut each half into three wedges. Pull the outer layers from each wedge and set aside for later. Thinly slice the inner core pieces and add to the steak. Lightly crush 2 garlic cloves with a knife and add to the steak along with half of the thyme sprigs.
In a mixing bowl, combine 3 tbsp brown sugar, 2 tbsp malt vinegar, 1 tsp Worcestershire sauce, 1 1/2 tsp kosher salt, and 1/2 tsp pepper. Whisk together and then slowly add one can of stout. Pour the marinade over the steak and seal the bag. Let the steak marinate for 2 to 4 hours. If using wooden skewers, soak them for at least 1 hour before using.
In a medium saucepan melt the butter over medium heat. Add the remaining can of stout along with the remaining 4 tbsp brown sugar, 2 tbsp malt vinegar, 1 tsp Worcestershire sauce, onion powder, 1 tsp kosher salt, and 1/2 tsp pepper. Lightly crush the remaining garlic clove with a large knife and add to the saucepan along with the remaining thyme sprigs. Bring to a boil and then reduce the heat to low. Simmer until the liquid is reduced to 1 cup.
In a small cup, combine the cornstarch and 2 tsp water to make a slurry. Remove the garlic clove and thyme sprigs. Whisk the slurry into the glaze, bringing the mixture back up to a boil to thicken. Remove from the heat.
Cut the reserved onion wedges in half if the pieces are large and separate the layers. Remove the beef from the marinade and pat the pieces dry with paper towel. Thread a steak cube onto one of the skewers followed by two onion pieces. Repeat 3 or 4 times to fill up the skewer, beginning and ending with the steak. Repeat until all the steak and onion is used. Drizzle the skewers with a little olive oil, rubbing to coat the pieces.
Preheat the grill to about 400°F. Spray the grill with non-stick spray. Place the skewers on the grill and cook for 3 to 4 minutes. Flip the skewers and cook for another 3 to 4 minutes. When the skewers are almost done, brush the top side with some of the glaze and cook for another minute. Flip the skewers and brush the other side with some of the glaze and cook for another minute until the steak is done to your liking. Remove from the heat.
Brush the skewers with more of the glaze and arrange on a platter to serve. If desired, garnish with fresh parsley. Serve the remaining stout glaze on the side. Enjoy!