The nano era demands the synthesis of new nanostructured materials, if possible by simplified techniques, with remarkable properties and versatile applications. Here, we demonstrate a new single-step reproducible melt-quench methodology to fabricate core-shell bimetallic (Au⁰–Ag⁰) nanoparticles (28–89 nm) embedded glasses (dielectrics) by the use of a new reducing glass matrix, K₂O–B₂O₃–Sb₂O₃ (KBS) without applying any external reducing agent or multiple processing steps. The surface plasmon resonance (SPR) band of these nanocomposites embedded in KBS glass is tunable in the range 554-681 nm. More remarkably, taking advantage of the selective reduction capability of Sb₂O₃, this single-step methodology is used to fabricate inter-metallic: rare-earth ions co-embedded (Au–Ag: Sm³⁺) dielectric (glass)-based-dnanocomposites and study the effect of enhanced local field on the red upconversion fluorescence of Sm³⁺ ions at 636 nm. The enhancement is found to be about 2 folds. This single-step in-situ selective reduction approach can be used to fabricate a variety of hybrid-nanocomposite devices for laser based applications (see supplementary information).