Cover the bottom of a skillet with coconut oil; warm over medium heat.
Add onion, serrano pepper, and garlic; cook and stir until onion is lightly browned, about 10 minutes.
Add tofu; stir to coat.
Stir cilantro stems, 1/4 package taco seasoning, 1 teaspoon cumin, and 1 teaspoon coriander into tofu mixture; cook and stir until fragrant and tofu is cooked through, about 5 minutes.
Add mangos and 1/4 package taco seasoning; cook, waiting 3 to 5 minutes before stirring.
Add chili powder and salt; stir to coat. Season with more cumin and coriander if desired.
Simmer tofu-mango mixture until most of liquid has evaporated, about 10 more minutes. Stir in cilantro leaves 1 minute before serving.