To understand the mechanisms that drive planet formation, we must link the present-day atmospheric composition of exoplanets to their formation history. High resolution spectroscopy (HRS; R~100,000) is a powerful technique which can provide precise abundance ratios of key volatile molecules (e.g. C/O) in exoplanet atmospheres but has previously lacked access to the refractory content of long-period giant atmospheres, resulting in predictions of degenerate formation scenarios. Here, we present the first results from our CRIRES+/VLT survey of directly imaged companions at M-band wavelengths (3.5-5 microns), pioneering the widescale use of this spectral range. The M-band provides a unique sensitivity to gaseous SiO, which traces the rock/vapour accretion history of giant exoplanets across a wide range of temperatures. Through precise measurements of the refractory silicon abundance, we constrain the formation pathway of our first survey target, the young brown dwarf companion TWA 5B. First light ELT instrumentation (e.g. METIS) will observe at M-band wavelengths, and exploration of this new parameter space with HRS is vital preparation for the ELTs.