He went to Rome around 1639-1640 where he initially was permitted to live in the monastery of Santa Croce in Gerusalemme in Milan through the intercession of the abbot Don Ilarione Rancati. The abbot also was instrumental in securing Courtois' first official commissions, a large fresco of the miracle of the loaves and fishes in the refectory of the monastery (1641). In Rome he also became friends with Pieter van Laer, a Dutch genre painter active in Rome where he was known by the nickname 'Bamboccio'. Pieter van Laer was known for his genre scenes, animal paintings and landscapes, which included anecdotal scenes placed in the environs of Rome. The style of genre painting practiced by Pieter van Laer was followed by other Northern and Italian painters. These followers became known as the Bamboccianti and a painting in this style as a "Bambocciata" (plural: "Bambocciate"). Michelangelo Cerquozzi, the leading battle painter in Italy in the first decades of the 17th century who also painted genre paintings in the style of the Bamboccianti, recognized Courtois' talent and encouraged him to paint battle scenes.