1. Stir together first 7 ingredients.
2. Cut roast lengthwise, cutting to but not through other side. Open cut piece to enlarge roast; pound to 1/2-inch thickness, making a large rectangle. Sprinkle with salt and pepper. Spoon spinach mixture evenly over pork. Beginning with long side, roll jellyroll fashion, and tie securely with heavy string at 2-inch intervals. Place roast on a lightly greased rack in a shallow roasting pan.
3. Bake at 325° for 1 hour and 15 minutes or until a meat thermometer inserted into thickest portion registers 160°. Increase temperature to 375°, and bake 10 minutes or until browned. Remove from oven, and let stand 5 minutes. Remove string.
Note: To make ahead, bake roast, and cool. Wrap tightly in aluminum foil, and refrigerate. To serve, uncover and place roast in rack in a roasting pan; cover loosely with foil, and add water to pan. Bake at 375° for 45 minutes.