How to Make It
In a saucepan of boiling salted water, blanch the fava beans for 1 minute. Drain the beans and rinse them in cold water. Peel the favas. In another saucepan of boiling salted water, cook the asparagus, then the cauliflower, until just tender, about 3 minutes each. Drain, rinse in cold water and drain on paper towels.
In a large nonstick skillet, melt 3 tablespoons of the butter in the oil. Ad the lamb in batches, season with salt and piment d'Espelette and cook over moderately high heat until browned outside but still pink in the center, about 4 minutes per batch; transfer the lamb to a platter.
Add the pearl onions to the skillet and add enough water to cover them by half. Bring to a boil over high heat. Reduce the heat to moderately high and cook, stirring and shaking the pan often, until the water has evaporated and the onions are tender and slightly caramelized, 10 to 15 minutes.
Melt the remaining 3 tablespoons of butter in a large nonstick skillet over moderate heat. Add the chanterelles, season with salt and cook, stirring often, until lightly browned, about 3 minutes. Add all of the vegetables, including the tomatoes, to the skillet along with the lamb and chopped rosemary and season with salt and piment d'Espelette. Bring to a simmer and stir until heated through. Spoon the sauté onto plates, garnish with rosemary leaves and serve.