"You can only amputate the sick to a certain degree; if you amputate too much, you will kill the patient" says Dale Allison, another Jesus scholar, reacting to scholars in the line of Crossan and Marcus Borg, who have stripped the historical Jesus of his apocalipticism and jewishness, thus ignoring tons of ancient evidence. That Jesus was, for example, an apocalyptic prophet and an observant Jew is supported in the earliest layers of tradition, such as the Q gospel (50s CE), Mark (60s) and Paul (50s). This is not a problem for Crossan, who says that the apocaliptic material (the belief that the world was about to end) was added to the gospels by the early church soon after Jesus died. Of more historical value (at least for him) are documents like the late Epistle of Barbanas (100s), the Didache (70s), the Secret gospel of Mark (the earliest copy dating from the middle ages), the Gospel of Thomas (150s) and - how odd - the reliefs made in stone in the 3rd or 4th century that depict Jesus as a greek philospher. How far can you press your hypothesis in one direction?
Key to Crossan's method is the concept of multiple attestation. If one complex (for example, the relationship between children and the Kingdom of God) appears indeppendently in more than one source, then that complex goes back to the historical Jesus. I would have no problem with this if Crossan were consistent about his own methods. Other multiple attested complexes and events, such as there being a group of twelve apostles, or the passion narrative, or the words of Jesus at the last supper, or the so-called nature miracles, he simply says "they are inventions". On the other hand, some sayings appearing in only one source ("I will destroy this house...", in the gospel of Thomas) he considers authentic.
Despite the evidence, in multiple independent sources, that there was a last supper (Paul, Mark, John) Crossan calls it an invention just because it isnt mentioned in a 1st century text known as Didache. If it isnt in the Didahce, then it never happened. (!)
This doesnt mean that Crossan is always far from the historical Jesus. The idea of a free exchange of food and miracles at the very roots of the Jesus movement, open commensality and radical egalitarianism must be very close to what actually happened. But as one reviewer put it, many, many pieces of the puzzle are missing... or have been ignored on purpose.
For more on the subject, I strongly recommend N.T. Wright's Jesus and the Victory of God.