Thank you for your question about planting onions in a new asparagus bed, are they compatible during the first year?  I have included, ‘Vegetable Garden Hints’, which discusses the culture of onions and asparagus.  Both plants enjoy rich soil high in organic matter but do not compete well with weeds or other crops for water nutrients and space.   I would recommend that these plants each have their own space.

https://cmg.extension.colostate.edu/Gardennotes/719.pdf

Asparagus

“Soils – Asparagus tolerates a wide range of soils as long as they are well drained.

It prefers soil high in organic matter, and full sun (eight hours/day

minimum).

Mulching – Asparagus competes poorly with weeds and other crops for water,

nutrients, and space.”

Onions

“– The onion family has a poor, inefficient root system, making the crop

intolerant of poor soils and competition from weeds. The plants are heavy

feeders. Quality produce arises from a well-drained, fertile soil, rich in

organic matter.”