Designing a carbon coated core@shell structure (M@C) is a popular and effective strategy to boost catalytic performance. However, most synthesis technologies for M@C reported up to now are relatively complicated and time consuming (multiple steps for at least a few hours). Herein, a rapid one-step process (4 min only), plasma electrolytic deposition, was developed to fabricate CoMo/CoMoO_x@C with a Janus metal/oxide core in a carbon shell, which realized a significantly enhanced hydrogen evolution reaction (HER). More interestingly, this synthesis method is extendable to many other metals. The resulting material can be easily optimized by tuning the metal/oxide core and/or the carbon shell through a wide range of parameters. NiCoMo/NiCoMoO_x@C was better than commercial Pt/C in HER catalysis, and NiFeMo/NiFeMoO_x was among the best overall water splitting catalysts reported very recently. This work presents a fast and general technique for the design of electrocatalysts with intriguing properties.