A girl in a flowing long gown with long blonde hair and a magically flowing voice cast a spell at the Cellar Door Monday evening that will not soon be forgot. Her name is Joni Mitchell.

It's not a well known name--yet--and what acclaim she has gathered has been mostly for the songs she has written; songs such as "Both Sides Now," "The Circle Game," "Michael From Mountains." They are good songs, so good that her writing is enough. It really isn't necessary that she be a fine performer.

But she is a marvelous performer, shyly yet firmly brining the audience to her with the help of a sensitively played guitar and an exquisite, sweet voice. She has the same gentle power to move people and stir emotions that Judy Collins and Joan Baez possess.

Her's is not a nightclub set in the accepted sense. It is a concert, and not even Vladimir Horowitz commanded a more rapt or appreciative audience. Yet the informatlity of Miss Mitchell's discourse between songs (like Miss Collins she has a winning way of seeming to share spontaneous thoughts with her audience) and the joy she can evoke in melody and lyric line prevent the evening from becoming a somber ceremony.

Miss Mitchell is a Canadian girl in her mid-20s who has been living in California for the past three years. Her music is tuneful and fresh, often in counterpoint to the story her lyrics are telling. In this adult age of folk music, inner struggles to find reason and meaning lin life is a prominent theme. The setting varies from city to country, from bedroom to airport. Miss Mitchell uses the city as a backdrop ("Chelsea Morning") or as a signpost (the red and green of "Marcy"). She talks about discontent in the city ("Nathan LaFraneer") and restlessness in the country ("Urge for Going").

As you may have sensed, Joni Mitchell is not a girl for foolishness. Even her most casual songs are in the light heavyweight division. But she has humor--as, for example, in "Good Mornin', Morgantown" and she refused to take herself too seriously on stage.

Her soprano is clear and graceful. She is well acquainted with its range and limitations, working carefully within them. Nor should her musicianship be slighted. She produces guitar chords that blend excitingly with her voice and confidently establishes and underlines mood with the instrument. In truth, her mature professionalism is more than a little astonishing.

Her songs and her thoughtful poetry won't be for every taste. Her only album so far, titled "Joni Mitchell" and released by Reprise, poked its head tentatively onto the best seller charts and, as though not feeling comfortable with the other kids in the neighborhood, withdrew. There is also some question of whether the delicate quality she conveys can be projected into a larger, less friendly settling.

Still, that isn't at issue here. The Cellar Door is an ideal showcase for Miss Mitchell. Her voice carries well, even when she is talking half to herself between songs, and the lighting engineer has come up with a bonus for balcony viewers in a surrealistic blob of blue light that dances in her hair.

There is no need to urge people to see Miss Mitchell. The Cellar Door was full for both shows Monday evening and will doubtless be crowded through Saturday. But if you can make a reservation, you should.