The paper presents an ultra-simplified computational spectrometer that employs a one-to-broadband diffraction decomposition strategy, utilizing a numerical regularized transform based on the spectrum of diffracted radiation. The key innovation is the use of a simple, arbitrarily shaped pinhole as the partial disperser, eliminating the need for complex encoding designs and full spectrum calibration. This design achieves a reconstructed spectral peak location accuracy better than 1 nm over a 200 nm bandwidth and excellent resolution for peaks separated by 3 nm in a bimodal spectrum, all within a compact footprint of under half an inch. The approach also enables broadband coherent diffractive imaging without requiring prior knowledge of the broadband illumination spectrum, assumptions of non-dispersive specimens, or correction for detector quantum efficiency. The experimental validation demonstrates the effectiveness of the proposed spectrometer in various applications, including broadband CDI, with high robustness, low cost, and long-term stability.The paper presents an ultra-simplified computational spectrometer that employs a one-to-broadband diffraction decomposition strategy, utilizing a numerical regularized transform based on the spectrum of diffracted radiation. The key innovation is the use of a simple, arbitrarily shaped pinhole as the partial disperser, eliminating the need for complex encoding designs and full spectrum calibration. This design achieves a reconstructed spectral peak location accuracy better than 1 nm over a 200 nm bandwidth and excellent resolution for peaks separated by 3 nm in a bimodal spectrum, all within a compact footprint of under half an inch. The approach also enables broadband coherent diffractive imaging without requiring prior knowledge of the broadband illumination spectrum, assumptions of non-dispersive specimens, or correction for detector quantum efficiency. The experimental validation demonstrates the effectiveness of the proposed spectrometer in various applications, including broadband CDI, with high robustness, low cost, and long-term stability.