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We investigate the effect of a lower index oxide layer inclusion within a highly doped silica glass slot waveguide for optimized supercontinuum generation at telecom wavelengths. By controlling the thickness of the oxide slot, we demonstrate that one can engineer the waveguide dispersion profile so that to obtain supercontinua with vastly different spectral broadening dynamics and bandwidths. Using this approach, we designed and fabricated a waveguide with a low and flat dispersion profile of less than 43 ps/km/nm across a wavelength range spanning over 1000 nm. We show that, when pumped at the telecom C-band, we can generate a supercontinuum that spans over 1.5 octave, from approximately 817 nm to 2183 nm. The numerical simulations, whose parameters are derived from the measured waveguide dimension and material indices, exhibit good agreement with experimental measurements, where one can observe both a qualitative and quantitative match in the supercontinuum overall spectrum and specific features (e.g. soliton and dispersive wave locations). This study represents an important step forward in the control and manipulation of dispersive and nonlinear dynamics in highly doped silica glass waveguides, paving the way towards advanced on-chip broadband light manipulation.