How to Make It
In a medium skillet, heat 2 tablespoons of the olive oil until shimmering. Add the minced onion and cook over moderately high heat, stirring occasionally, until softened and starting to brown, 5 minutes. Stir in the minced garlic, the fennel seeds, mustard powder, crushed red pepper, coriander and chile powder and cook, stirring, until fragrant, about 2 minutes; let cool.
In a large bowl, mix the cooled onion mixture with the veal, bread crumbs, cream, egg and salt. Form into 1 1/2-inch meatballs and transfer to a rimmed baking sheet.
In a large skillet, heat the remaining 2 tablespoons of olive oil until shimmering. Add the meatballs and cook over moderately high heat, turning, until browned all over, 5 minutes. Gently push the meatballs to one side of the skillet. Spoon off all but 2 tablespoons of the fat from the pan, then add the mustard greens and stock. Cover and cook over moderate heat until the greens are wilted, 4 minutes. Spoon the meatballs and greens into bowls and serve.