The mid-infrared was the first spectral region to provide the direct emission spectrum of an exoplanet (Janson+ 2010). Since then, no other mid-infrared spectrum at medium-to-high spectral resolution has been published, partly because of the high sky background and the scarcity of mid-infrared AO-assisted spectrographs. The mid-infrared is yet a rich spectral window for exoplanetary atmospheres, in particular to study cloud coverage and disequilibrium chemistry, or to distinguish between protoplanet atmospheres and dust shells. To remedy this situation, we developed a new observing strategy using the VLTI/MATISSE interferometer and its new GRA4MAT combination with GRAVITY. We present here our successful observation of Beta Pictoris b and the planet's first medium-resolution L&M-band spectrum. Given its very high spatial resolution, MATISSE provides a great complementarity to the JWST for the planets at lowest separations (<0.4”). Its detection limits will further improve with the commissioning of the new VLTI AO system in 2024.