Melt butter in a pot over medium heat. Add onion; cook and stir until translucent, about 5 minutes. Mix in couscous; cook and stir until browned, about 5 minutes.
Stir in water, zucchini, potato, green beans, carrots, tomato puree, coriander, chile powder, cumin, and salt; bring to a boil. Simmer until tender, about 20 minutes. Stir in milk.
