Preheat oven to 225°F.
Add the mushrooms to a small baking dish or loaf pan, toss with salt. Add the sprigs of rosemary, oregano, and thyme. Drizzle with oil until about halfway covered (mushrooms will shrink as they cook).
Bake, uncovered, for 15 minutes. Stir and continue to cook for 45 more minutes. Remove and discard the sprigs of herbs. Remove the mushrooms from the oil.
Peel the potatoes and cut into ½ inch wide strips.
Stir together the cornstarch and beer into a slurry mixture. Add the sweet potatoes into the mixture, toss to coat.
Heat about 1 inch of oil in a wide pan until a drop of water pops, lower heat to just higher than medium.
Add the potatoes a few at a time making sure they aren’t touching. Turn over with tongs, cooking on all sides until browned and crisp, remove and allow to drain on a stack of paper towels.
Stir together the crema, garlic powder, smoked paprika, and salt.
Fill the tortillas with sweet potatoes and mushrooms, drizzle with sauce, top with green onions.