Aster is a Partner in Addleshaw Goddard's Professional Practices Group. He is recognised as a leading adviser to professional services firms on transactions, advisory matters and partner disputes.

The Legal 500 cites Aster as a "leading individual" and has said that he has "great technical expertise", "clearly knows his stuff, is friendly, responsive and solution-oriented". Chambers ranks him in Band 1 for non-contentious partnership work and quoted a client as saying: "He is superb, everything that a trusted adviser should be: commercial, responsive and a pleasure to work with." Aster was included in The Lawyer's Hot 100 for 2017.

Aster's transactional work includes mergers, acquisitions, disposals, joint ventures and LLP conversions. His advisory practice encompasses partnership agreements, governance, risk management and structuring issues. He has a particular interest in international structuring, including the establishment and operation of network structures (such as the Swiss Verein). On the contentious side, he assists firms and individuals with partner disputes, including team moves.

As well as professional services firms, Aster works with businesses in the financial services sector, including hedge funds and private equity houses, on the use of LLPs and partnerships.