Preheat oven to 400 degrees. Meanwhile, heat 2 tablespoons of olive oil in a large, deep skillet over medium-high heat. Add onion, celery, carrots, potatoes and mushrooms. Sprinkle the mixture with garlic powder, salt, rosemary and thyme. Cook, stirring often, for about 10-15 minutes until all the vegetables are tender and the potatoes begin to soften.
Sprinkle the flour evenly over the mixture and carefully toss to coat, cooking for about a minute. Pour in the Guinness, scraping up any bits stuck to the pan. Pour in the vegetable broth and stir to combine.
Let the mixture simmer for about 5-7 minutes or until the liquid thickens up and starts to look more like a sauce or a gravy and the potatoes are fork tender. Meanwhile, in a medium bowl, whisk together the flour, baking powder, salt, garlic powder and cheese.
Create a well in the center of the flour mixture and pour in the half and half, vegetable broth and butter. Mix until everything is combined and a sticky dough is formed -- do not overwork the mixture, just ensure that there are no dry spots.
Plop little mounds of dough over the top of the mixture in the skillet. It's ok if there are gaps in between, they'll grow a bit as they bake. Brush the tops of the dough balls with the remainder of the melted butter you've set aside.
Bake for about 20-23 minutes or until the biscuits have set and the stew is bubbly around the edges. Remove from oven and sprinkle with chopped parsley before serving, if desired.