The phrase "at once plain" is correct and usable in written English.
It can be used to describe something that is immediately clear or obvious without any ambiguity.
Example: "The instructions were at once plain, leaving no room for confusion about what needed to be done."
Alternatives: "immediately clear" or "instantly obvious".
It's at once plain-spoken and otherwordly, grounded in a vanishing rural way of life and uncannily modern.
Here he pays homage to the sweep of those Romantic works with an unabashed valentine to Ms. Mutter and her instrument, at once plain-spoken and defined by grand gestures.
It manages to be at once starkly plain and extremely uninteresting, boringly unadorned and yet oddly difficult to say.
What makes him a Ruizian figure is that his experience is at once elusive and plain, enacted with charming nonchalance even in its craziest moments.
It's an insight, and an interesting one for a political character who was at once direct and plain speaking – yet often frustratingly enigmatic, a chameleon who would fade in and out of sight.
She even lets out a little yawn in the video, which has been viewed more than 4 million times so far, because feeling that many emotions all at once is just plain exhausting.
Children as young as 5 ride in races that can be dangerous, with hundreds of horses thundering across the open plain at once, running at speeds of 30 miles per hour or more.
The figures are at once hidden and in plain sight.
In the middle of the back was a single pinched pleat, making the plain coat at once strange and wonderful.
#proud daddy ☺️ The comments of Mr Bieber - at once embarrassing, creepy and just plain strange - were almost as awkward as the New York Daily News's high-minded feature that included full-frontal photos of the 21-year-old singer while he was on holiday in Bora Bora, in the South Pacific.
