In order to attempt a definition of Essential Zen it is necessary first to provide a definition of Zen itself.
Wikipedia wrote:Zen is a school of Mahāyāna Buddhism, translated from the Chinese word Chán. This word is in turn derived from the Sanskrit dhyāna, which means "meditation".
Zen emphasizes experiential prajñā, particularly as realized in the form of meditation, in the attainment of enlightenment. As such, it de-emphasizes theoretical knowledge in favor of direct, experiential realization through meditation and dharma practice.
The establishment of Zen is traditionally credited to be in China, the Shaolin Temple, by the Southern Indian Pallava prince-turned-monk Bodhidharma, who came to China to teach a "special transmission outside scriptures" which "did not stand upon words". The emergence of Zen as a distinct school of Buddhism was first documented in China in the 7th century AD. It is thought to have developed as an amalgam of various currents in Mahāyāna Buddhist thought — among them the Yogācāra and Mādhyamaka philosophies and the Prajñāpāramitā literature — and of local traditions in China, particularly Taoism and Huáyán Buddhism. From China Zen subsequently spread south to Vietnam, and east to Korea and Japan.
Traditional Zen, or perhaps Conventional Zen then is a course of study and meditation, which if formally undertaken, will occur at a Zen monastery under the guidance of a previously acknowledged "Master" acting as abbot. The goal is to achieve enlightenment, and to have that enlightenment validated by the master.
So, what IS Essential Zen? As with other esoteric teachings, I believe it gets described and defined in the poetry associated with it. That includes the vignettes of those who have achieved, who could be called adepts, who could be called enlightened masters: that is, those who have achieved the goal as described by don Juan: "..what matters to a warrior is arriving at the totality of oneself."
Essential Zen is a thought process, not requiring meditation, not requiring monasteries, not requiring the approval of an acknowledged master. It is understood by recognizing the applicability of the teachings in everyday living. It is truly the lazy and indulgent man's way to enlightenment.