A practical, computationally efficient technique to EM-based design of single-layer SIW interconnects with microstrip transitions is presented in this work. The design technique efficiently exploits a simplified EM model of the SIW interconnect (surrogate model). The initial design of the SIW structure is obtained by using available empirical equations. A low-resolution EM model with solid walls (grooves) is proposed as an efficient SIW surrogate model for direct EM optimization using a few frequency points. The complete EM optimization of the surrogate model consumes much less time (< 20%) than just one simulation of the original EM model (with vias) using a high-resolution grid. The proposed practical technique is illustrated by designing a 10-50 GHz SIW interconnect with microstrip transitions on a standard FR4-based substrate.