To make the gravy, place the sweet potatoes in a 1½-quart pot. Add enough water to cover the potatoes and bring it to a boil. Cook the potatoes until fork-tender, 8 to 10 minutes. Set a colander or sieve over a heatproof measuring cup, then drain the potatoes, reserving the water in the measuring cup. Note the amount. Mash the potatoes into a fine paste.
Place the shallot, garlic, galangal, and ginger in a high-speed blender. Blend into a fine paste, adding 1 to 2 tablespoons of water if needed.
Heat a large wok over medium-low heat. Add the oil, then slowly pour in the spice paste and add the lemongrass stalk. Cook the paste until it's aromatic and starts to dry up, 10 to 15 minutes.
Stir in the curry powder and fermented bean paste. Add the sweet potato blanching liquid from above, plus enough water to get 7 cups of liquid total (so, if you had 4 cups of blanching liquid, you’d add 3 cups of water).
Add the mashed sweet potatoes and increase the heat to high. Stir in the palm sugar, ketchup, Yondu, salt, and mushroom seasoning. Cook the mixture until the flavors meld and the sugar has dissolved, 8 to 12 minutes. Taste and season accordingly. Remove the lemongrass stalk. (Note: If you’d like a smoother gravy, at this point you can transfer the mixture to a high-speed blender and blend.)
Meanwhile, fill a large pot with water and bring it to a boil. Blanch the mung bean sprouts for 2 to 3 minutes, until they are as crisp as you like, and use a slotted spoon to transfer to a bowl. Now, using the same water, cook the noodles according to the package instructions, then strain and rinse.
Divide the noodles between 4 bowls and ladle the hot gravy on top. Top with the mung bean sprouts, tofu, cruller, jalapeño, shallots, cilantro, and scallions. Serve right away with a squeeze of lime juice.