In a large nonstick skillet, heat the oil or lard over medium heat and fry the onion and oregano.
Cook, stirring until the onion begins to brown, about 5 minutes.
Add the mushrooms and cook, partially covered, stirring occasionally, until the mushrooms are tender and starting to brown, 8 to 10 minutes.
Add the remaining ingredients.
Raise the heat and cook until completely heated through, 3 to 4 minutes.