A flow-through extraction system based on an immobilized hydrophobic deep eutectic solvent (HDES) was developed as a phase-engineered platform for the on-site separation and preconcentration of polycyclic aromatic hydrocarbons (PAHs) from aqueous matrices. The system employs a menthol:1-hexanol (1:2, mol/mol) deep eutectic solvent confined within a porous polytetrafluoroethylene (PTFE) support inside a pipette-tip device, forming a stable hydrophobic liquid phase that enables partitioning-driven transfer of analytes under dynamic flow conditions. During operation, large sample volumes are processed directly at the sampling site, where PAHs are continuously transferred from the aqueous phase into the immobilized DES via interfacial mass transfer. This configuration ensures rapid enrichment and immediate stabilization of hydrophobic analytes, minimizing losses associated with sample transport and storage. The extraction performance was governed by the combined effects of phase stability, analyte distribution into the confined liquid phase, and residence time under flow-through conditions. Under optimized parameters, processing of 1 L water samples resulted in enrichment factors of 440–1600, while the limits of detection for the individual target PAHs ranged from 1 to 60 ng L−1, with satisfactory precision (RSD ≤ 13%). The system demonstrated negligible matrix effects and consistent performance during repeated use of the PTFE carrier with fresh DES immobilization before each extraction cycle. The method was validated against the ISO 17993:2002 liquid–liquid extraction procedure, showing statistically equivalent results while significantly reducing solvent consumption and simplifying the workflow. Sustainability assessment using the Green Analytical Procedure Index (GAPI) and Analytical GREEnness (AGREE) metrics (AGREE score 0.57) confirmed improved environmental performance due to the absence of bulk organic solvent during on-site extraction and reduced waste generation. The proposed approach represents a new class of supported liquid-phase extraction systems that integrate sampling, phase separation, and preconcentration into a single step. This platform provides efficient mass transfer, high enrichment capability, and operational robustness, offering a promising tool for field monitoring of hydrophobic organic pollutants in water.