The inner few au region of protoplanetary disks is the cradle of terrestial planets. Looking at this region provides insights into the early phase of planet formation: dust grain growth and coagulation. Interferometers working at mid-infrared wavelengths can resolve the thermal radiation from the warm inner regions, providing information on the structure of these disks at very high resolution. I will present early results from the guaranteed observing program for young stellar objects of MATISSE, the new VLTI instrument for the thermal infrared. One highlight is that based on an ongoing survey of Herbig AeBe stars, the L band (3 micron) brightness distribution of these disks is commonly found to be asymmetric (one-third of a sample of 21 observed systems). I will discuss the potential sources for the asymmetry, like binarity, illumination effects, or asymmetric disk structure, specifically addressing the case of HD 163296.