Is this plant lambs quarters or another weed or a flower?
Does not offhand look like lamb's quarters, too curly, no silver dusting visible. I suppose it might be some variation, but I doubt it.
Something in the Brassica family. Not Chenopodium album.
Given this is a flower garden would this be a weed?
Did you ever grow any veggies there such as Oriental greens? A weed is in the eye of the beholder. You must decide if you want it or not.