Cut chicken into 1/2-inch pieces. Puree in processor in 2 batches to very smooth paste. Refrigerate until well chilled, at least 1 hour.
Meanwhile, thinly slice leek and shallot. Melt butter in heavy small skillet over low heat. Add leek and shallot and cook until soft, stirring occasionally, about 15 minutes. Cool.
Combine half of chicken with half of leek mixture in processor. Puree until smooth. Add half of egg whites and process until smooth. Transfer to bowl. Repeat with remaining chicken, leek mixture and egg whites. Refrigerate until well chilled.
Blend half of chicken mixture, salt, pepper and nutmeg in processor. With machine running, slowly pour 1 cup plus 2 tablespoons cream through feed tube. Blend 1 minute. Transfer to large bowl. Repeat with remaining chicken, salt, pepper, nutmeg and 1 cup plus 2 tablespoons cream. Combine both chicken mixtures.
Bring chicken broth to boil in large skillet. Add spinach and basil and toss until just wilted, about 45 seconds. Drain well. Squeeze out any remaining liquid. Puree spinach mixture with remaining 3/4 cup cream until smooth. Blend in 1/4 of chicken mixture.
Preheat oven to 350 deg. Line 9x5-inch loaf pan with plastic wrap, leaving 5-inch overhang. Pour half of white chicken mixture into pan. Cover with half of spinach mixture. Using knife, swirl through layers to create marble effect. Repeat with remaining chicken and spinach. Tap pan on work surface to settle pate. Fold overlapping plastic over top of pate. Cover pan tightly with foil. Place loaf pan in deep roasting pan. Pour enough water into roasting pan to come 2/3 up sides of loaf pan. Transfer to oven and bake 1 1/4 hours. Cool. Weight pate with heavy object and refrigerate overnight.
Unmold pate onto platter. Let stand at room temperature 1 hour before serving. Pass Tomato & Garlic Sauce separately.