Preheat oven to 425°F.
Peel and cut the sweet potatoes into chunks, add to a large pot of salted boiling water. Cook until fork tender, drain and return to pot.
Add the almond milk, salt, and garlic powder. Mash until well combined.
Heat the olive oil over medium high heat in a large skillet. Add the leeks, carrots and celery, cooking until softened.
Add the mushrooms, cook until darkened and softened.
Stir in the sage, rosemary, salt, pepper, garlic and onion powder. Sprinkle with flour, stir until the flour has been moistened. Pour in the beer, scraping to deglaze the pan.
Stir in the corn and vegetable broth. Simmer until thickened.
Pour into am 8x8 pan. Spread the sweet potatoes over the top of the pan.
Bake until filling is bubbly, about 15 minutes.
Serve warm.