Place the potatoes in a medium saucepan and add water to cover. Bring to a boil, then reduce the heat to medium-low and simmer, covered, until the potatoes are tender when pierced with the tip of a sharp knife, 10 to 12 minutes. Drain well and set aside.
If you’re going to bake the tortillas, preheat the oven to 375.
Squeeze out as much liquid from the spinach as you can.
In a large skillet, cook the onion and poblano pepper over medium heat, stirring occasionally and adding water 1 to 2 tablespoons at a time as needed to keep the vegetables from sticking, until softened, 7 to 8 minutes. Add the garlic and cumin and cook until fragrant, about 1 minute.
Add the reserved spinach and potatoes along with the plant milk and nutritional yeast to the skillet. Season to taste with salt and pepper and cook until heated through, 2 to 3 minutes. Remove from the heat and set aside.
To make taco shells, drape each corn tortilla over the rungs of an oven rack and bake for 7-8 minutes at 375 until they are crispy and golden.
To serve, place the tortillas on individual serving plates or a large platter and divide the potato mixture among them, spooning it onto the center of each.
Sprinkle with cilantro and serve.
