Optical frequency combs, typically produced by mode locked lasers, have revolutionized many applications in science and technology. Frequency combs were recently generated by micro resonators through nonlinear Kerr processes. However, the comb span from micro resonators was found to be limited by resonator dispersion and mode spectrum. While dispersion engineering has been reported in on-chip devices, monolithic crystalline resonators offer an advantage of high optical quality factor. Moreover, most resonators used for comb generation support many mode families, leading to unavoidable crossings in resonator spectrum. Such crossings strongly influence comb dynamics and may prevent stable coherent mode-locking and soliton states. We report a new crystalline resonator approach supporting dispersion control and single mode spectrum while maintaining high quality factor. Dispersion engineering by waveguide micro-structuring is used to flatten the dispersion in our MgF2 resonator. Both absolute magnitude of dispersion and its slopes can be altered over a wavelength span exceeding an octave. Dispersion flattening leads to generation of an octave-spanning frequency comb with repetition rate of 46 GHz and coupled pump power below 100 mW. We also demonstrate that the micro- structuring dispersion engineering approach can be used to achieve flattened and anomalous dispersion in a CaF2 resonator near 1550 nm wavelength. In addition, we describe observation of discrete steps between the modulation instability states of the primary comb and on the three-stage comb unfolding dynamics. The micro-structured resonators may enable efficient low repetition rate coherent octave spanning frequency combs without external broadening, ideal for applications in optical frequency synthesis, metrology, spectroscopy, and communications.