Quasinormal modes of a black hole are closely related to the dynamics of the spacetime near the horizon. In this connection, the black hole ringdown phase is a powerful probe into the nature of gravity. However, the challenge of computing quasinormal mode frequencies has meant that ringdown tests of gravity have largely remained model-independent. In this talk, I will introduce Metric pErTuRbations wIth speCtral methodS (METRICS) [1], a novel spectral scheme capable of accurately computing the quasinormal mode frequencies of black holes, including those with modifications beyond Einstein's theory or the presence of matter. I will demonstrate METRICS' accuracy in calculating quasinormal mode frequencies within general relativity, as a validation, and its application to Einstein-scalar-Gauss-Bonnet gravity [2, 3], an example of modified gravity theory to which METRICS has been applied. I will also present preliminary results from applying METRICS to dynamical Chern-Simons gravity. Finally, I will discuss potential future applications of METRICS beyond computing black hole quasinormal modes. [1]: https://arxiv.org/abs/2312.08435 [2]: https://arxiv.org/abs/2405.12280 [3]: https://arxiv.org/abs/2406.11986