In praise of... My frond-y flower

Despite recent heavy rain, its legacy still evident in puddles and sodden ground, we are currently enjoying a St Martin's Summer. Yes, the signs of autumn are all around us; our leaves are gold, russet and fiery red; but our days are still mild enough for light jackets, and there are still a few vestiges of summer in our gardens. And in my garden a strange, frond-y thing is in full bloom.

Several months ago I sowed the contents of a small pack of seeds, which held the promise of a variety of unspecified bee-friendly flowers. Among the flowers which duly appeared were several love-in-a-mist, or nigella, with their distinctive, delicate, feathery leaves. So, when a feathery stalk appeared several weeks ago, that's what I assumed it was... except that it started growing a lot taller than any love-in-a-mist I've ever seen. For a long time that's all it was; curly, intricate, feathery fronds, reaching upwards, in search of sunshine and sky. For a while it bobbed and it bowed, sweeping elegantly downwards, until I found a stick with which to support it. Its head had become weighty with buds; these, somewhat surprisingly, flowered into nothing like nigella, and everything like asters! 

If I could pronounce the word I'd have christened this plant Quetzalcóatl, in recognition of its stalk's resemblance to a feathered serpent; instead, I just call it my frond-y flower. 

So, what species of plant is this three-foot tall intricately plumed stalk currently topped with three flowers from the aster family? Is it some kind of hybrid, or an actual plant I've just never noticed before? (That, of course, is entirely possible - would I have noticed it in a full flower-bed in July? Or only now, because it's sprouting somewhat incongruously and solitarily in November?). I have searched online, and consulted a green-fingered sister, who is equally puzzled. All I know is: it is rather lovely, albeit somewhat foolhardy to be bursting into life in mid-November; it is pleasingly incongruous, utterly welcome... and it has what look like two more tiny buds near the top of its stalk!