We report the generation of boron nitride nano-onions (BNNOs) from pure hexagonal-BN crystallite powder in vacuum, via a one-pot, non-toxic, catalyst-free, rapid and potentially scalable high-temperature lamp ablation procedure. An array of characterization procedures revealed nanoparticle (a) shapes and sizes, morphing from polyhedral for diameters of order 101 nm to quasi-spherical at diameters of order 102 nm, and (b) composition, with a 1:1 B:N ratio while also confirming the absence of contaminants. A formation mechanism is proposed whereby BNNOs evolve from the initial thermal exfoliation of the bulk precursor powder into nano-platelets. They fold and close into hollow nano-cages that are more stable thermodynamically. Reactor temperatures were measured to be ∼ 1500–1600 K for experiments yielding the largest amounts of BNNOs. A relatively narrow range of ∼ 1300–1800 K can be established for viable BNNO generation. Structural modeling sheds light on the relative stability of the nanostructures as a function of shape, size and number of layers.