Modern versions of the Miller-Urey experiment claim that formamide (NH_2_CHO) could be the starting point for the formation of metabolic and genetic macromolecules. Intriguingly, formamide is indeed observed in regions forming Solar-type stars as well as in external galaxies. How NH_2_CHO is formed has been a puzzle for decades: our goal is to contribute to the hotly debated question of whether formamide is mostly formed via gas-phase or grain surface chemistry. We used the NOEMA interferometer to image NH_2_CHO towards the L1157-B1 blue-shifted shock, a well known interstellar laboratory, to study how the components of dust mantles and cores released into the gas phase triggers the formation of formamide. Results. We report the first spatially resolved image (size ~9", ~2300AU) of formamide emission in a shocked region around a Sun-like protostar: the line profiles are blueshifted and have a FWHM~=5km/s. A column density of N(NH_2_CHO)=8x10^12^cm^-1^, and an abundance (with respect to H-nuclei) of 4x10^-9^ are derived. We show a spatial segregation of formamide with respect to other organic species. Our observations, coupled with a chemical modelling analysis, indicate that the formamide observed in L1157-B1 is formed by gas-phase chemical process, and not on grain surfaces as previously suggested. The SOLIS interferometric observations of formamide provide direct evidence that this potentially crucial brick of life is efficiently formed in the gas-phase around Sun-like protostars.