Place the poblano or hatch chiles on a wire rack directly over a gas burner or under a broiler set to 500°F (260°C). Turn regularly until the skins are evenly charred and blistered on all sides, 5-6 minutes.
Transfer the charred chiles to a bowl and cover tightly. Let them steam for 10 minutes to loosen the skins.
Peel away the skins, then make a small slit along one side of each chile and remove the seeds, keeping the chiles as intact as possible.
Carefully stuff each chile with the vegan cheese, pressing gently to close the opening.
In a large bowl, whisk together the all-purpose flour, masa harina, garlic powder, nutritional yeast, and salt. Gradually whisk in the plant-based milk, stirring until a smooth batter forms with a thick, pourable consistency.
Heat the frying oil in a deep skillet over medium heat. After 3–4 minutes, test the oil by dropping a small amount of batter into it; it should sizzle immediately.
Dip each stuffed chile into the batter, coating it fully, then carefully place it into the hot oil. Fry in batches over medium heat for 3–4 minutes per side, turning once, until golden brown and crisp.
Transfer the fried chiles to a wire rack suspended over a baking pan to drain excess oil.
To prepare the optional crema, combine the plant-based milk, garlic, cilantro, lime juice, lime zest, avocado, vegan yogurt, and salt in a blender. Blend until smooth.
Warm the salsa rojo or red chile sauce in a saucepan over medium heat until heated through.
To serve, spoon the warm sauce onto a plate, place the chiles rellenos on top.
Drizzle with cilantro lime crema and garnish with minced red onion, and cilantro leaves.