Taking @michalmuskala, @sribe and @hauleth advise and
looking around the inter-webs; came up with the below examples.
Hope it helps others.

# customer_controller.ex
def index(conn, _params) do
  query = "SELECT json_agg(customers)::text FROM customers"
  result = Repo.query!(query, [])

  conn
  |> put_resp_content_type("application/json")
  |> send_resp(200, ["{\"list\":", result.rows, "}"])
end

More complex example; return an invoice as json object with customer details and list of items bought

# invoice_controller.ex
def show(conn, %{"id" => id}) do
  query = """
  SELECT row_to_json(invoice)::text
  FROM (
    SELECT *,
    (
      SELECT row_to_json(c)
      FROM (
        SELECT *
        FROM customers
        WHERE invoices.customer_id = customers.id
      ) c
    ) AS customer,
    (
      SELECT json_agg(i)
      FROM (
        SELECT id, product_id, qty, rate, total
        FROM invoice_details
        WHERE invoice_id = $1::bigint
      ) i
    ) AS items
    FROM invoices
    WHERE id = $1::bigint
  ) invoice;
  """

  result = Repo.query!(query, [id])

  conn
  |> put_resp_content_type("application/json")
  |> send_resp(200, ["{\"data\":", result.rows, "}"])
end

Also, you don’t need the List.flatten(hd(result.rows)) part. The body passed to send_resp can be iodata, which, among others, means a nested list of binaries. send_resp(200, result.rows) should work just fine.

Also using SELECT json_agg(customers)::text FROM customers might improve things on the DB side.

Have you tried casting in the DB? Like:

query = "select array_to_json(array_agg(customers))::text from customers"
{:ok, result} = Repo.query(query, [])

conn
|> put_resp_content_type("application/json")
|> send_resp(200, List.flatten(hd(result.rows)))