Create an IAM role in Account B with the proper permissions for object storage and establish a trust relationship allowing the serverless function's role in Account A to assume this role.

Configure the object storage in Account B to be publicly accessible and regulate access using resource-based policies that check the request origin.

Generate access keys for a user in Account B, store them as environmental variables for the serverless function in Account A, and use these keys within the function to access the object storage.

Set up a role in Account B granting full access to the object storage and define a broad trust policy that permits the assumption of this role by other identities, relying on additional service-specific policies in Account A to enforce restrictions.