  Working with Hamiltonians from chiral effective field theory, we develop a
novel framework for describing arbitrary deformed medium-mass nuclei by
combining the in-medium similarity renormalization group with the generator
coordinate method. The approach leverages the ability of the first method to
capture dynamic correlations and the second to include collective correlations
without violating symmetries. We use our scheme to compute the matrix element
that governs the neutrinoless double beta decay of $^{48}$Ca to $^{48}$Ti, and
find it to have the value $0.61$, near or below the predictions of most
phenomenological methods. The result opens the door to ab initio calculations
of the matrix elements for the decay of heavier nuclei such as $^{76}$Ge,
$^{130}$Te, and $^{136}$Xe.
