How to Make It
In a large nonreactive skillet, heat the 2 tablespoons of oil. Add the pancetta and cook over moderate heat until crisp, about 3 minutes. Add the garlic and cook, stirring, until fragrant, about 1 minute. Add the arugula and cook, stirring, until wilted, 1 to 2 minutes. Transfer to a plate and let cool.
In a large bowl, combine the ground chicken, bread crumbs, cheese, capers, egg, 3/4 teaspoon of salt and 1/8 teaspoon of pepper. Add the arugula mixture and beat until well blended. Roll the mixture by teaspoonfuls into 3/4-inch balls.
In a large skillet, heat 1/4 inch of olive oil. Add half of the meatballs in a single layer and cook over moderate heat, turning, until browned all over, about 3 minutes. Using a slotted spoon, transfer the meatballs to a large plate. Discard the fat and wipe out the skillet.
In the same skillet, melt the butter over moderate heat. Add the shallots and cook until softened, about 3 minutes. Add the Cognac, raise the heat to high and cook until evaporated. Add the tomatoes and thyme and season with salt and pepper. Bring to a simmer and cook until the sauce is thick, about 8 minutes.
Add the meatballs to the sauce and simmer over low heat just until heated though, about 3 minutes. Serve immediately.