  Einstein gravity minimally coupled to a scalar field with a two-parameter
Higgs-like self-interaction in three spacetime dimensions is recast in terms of
a Chern-Simons form for the algebra $g^{+}\oplus g^{-}$ where, depending on the
sign of the self-interaction couplings, $g^{\pm}$ can be $so(2,2)$, $so(3,1)$
or $iso(2,1)$. The field equations can then be expressed through the field
strength of non-flat composite gauge fields, and conserved charges are readily
obtained from boundary terms in the action that agree with those of standard
Chern-Simons theory for pure gravity, but with non-flat connections. Regularity
of the fields then amounts to requiring the holonomy of the connections along
contractible cycles to be trivial. These conditions are automatically fulfilled
for the scalar soliton and allow to recover the Hawking temperature and
chemical potential in the case of the rotating hairy black holes presented
here, whose entropy can also be obtained by the same formula that holds in the
case of a pure Chern-Simons theory. In the conformal (Jordan) frame the theory
is described by General Relativity with cosmological constant conformally
coupled to a self-interacting scalar field, and its formulation in terms of a
Chern-Simons form for suitably composite gauge fields is also briefly
addressed.
