Roast the poblanos directly over the flame of a gas burner or under the broiler, turning with tongs, until charred all over, about 10 minutes. Transfer to a bowl, cover with a plate and let steam until cool enough to handle, about 10 minutes. Scrape off the charred skin with a paring knife; remove the stems and seeds. Cut the poblanos into strips and set aside.
Meanwhile, put the potatoes in a large nonstick skillet and cover with cold water; season with salt. Bring to a simmer over medium-high heat and cook until just tender, 5 to 6 minutes; drain. Wipe the skillet dry.
Heat the olive oil in the skillet over medium-high heat. Add the potatoes and cook until golden and crisp, about 4 minutes per side. Remove with a slotted spoon and drain on paper towels. While still hot, sprinkle with half of the herbs, and salt to taste. Discard the excess oil and wipe out the skillet.
Heat the butter in the skillet over medium-high heat. Add the onion and cook, stirring occasionally, until golden, about 6 minutes. Add the mushrooms and cook, stirring occasionally, until golden, 6 to 8 minutes. Stir in the remaining herbs and the garlic; cook 1 minute. Add the roasted poblanos, crema, lime juice and a few spoonfuls of water; stir until warmed through, 1 to 2 minutes. Season with salt and pepper. Serve the potatoes and mushroom mixture in tortillas with assorted toppings.