In a medium bowl stir together the gochujang, soy sauce, water, brown sugar, and rice vinegar. Stir until the gochujang and brown sugar are dissolved.
Clean each portobello mushroom to remove any dirt or debris. Cut each cap in half, then slice crosswise into thin strips.
Place the mushroom strips into a shallow dish and pour the gochujang marinade over top. Gently stir the mushrooms to coat in the marinade, taking care not to break them.
Let the mushrooms marinade for about 10 minutes, stirring occasionally.
Add the cooking oil to a large skillet and heat over medium. Once hot, add the sliced yellow onion and sauté for about two minutes, or just until the onion begins to soften.
Add the marinade mushrooms and all of the gochujang marinade to the skillet. Continue to cook the mushrooms and onion over medium until the mushrooms have softened and the liquid in the skillet reduces to a slightly thickened sauce (about 8 minutes).
DONE!
