What is wrong with "My grandmother has cooked mushrooms."?
If she 'has cooked' them succesfully, that means she "ugotowała" them. "gotowała" is imperfective.
Does it mean, that we do not know the result? That she was trying to cook them and maybe she finished cooking succesfully!?
She probably finished it, but it simply doesn't matter. It just says what she was doing at some point of time in the past. Or maybe she did it regularly, but then the translation should be "My grandmother used to cook mushrooms".
That is possibly the most Polish sentence in this course :D