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.
Using a spoon or a cookie scoop, 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.