To prepare chicken breasts: Lay chicken breasts flat on work surface. Using a sharp knife and starting at the edge of each breast, slice in half horizontally until about. three-quarters of the way through. Open the breast up like a book and lay flat to have one larger, thinner fillet. Sprinkle a small amount of water on a large piece of plastic wrap to keep chicken from sticking and place one breast in the center. Season with salt, then cover breast with plastic wrap and use a large kitchen mallet to gently pound and flatten the breast. Repeat process with the three remaining breasts. Wrap in plastic wrap and set aside in the refrigerator.To prepare tomato sauce: In a large saute pan over medium high heat, heat olive oil until hot. Add onions and cook until softened, 3 to 4 minutes. Add tomatoes, garlic, salt, and thyme sprigs. Cover saute pan and reduce heat to medium low. Continue to cook slowly, 15 to 20 minutes. Remove pan from heat, then remove thyme sprigs. Place tomato sauce in blender or food processor and blend until smooth. Strain tomato sauce through a fine-mesh sieve, into a medium saucepan over medium high heat. Add butter and whisk until butter is melted and sauce is smooth. Remove from heat.To prepare chicken mousse: Put the olives in a food processor and puree. Set aside. In a small bowl, soak the bread pieces in the milk. Place the chicken breast in the bowl of a food processor and chop. Add milk-soaked bread to chopped chicken, pulsing processor until mixture is combined. While motor is running, add cream, nutmeg, salt, and pepper. Process until combined. Mix one-third of this mixture with the olive puree and set aside. Mix remaining chicken mousse with herbs and divide into four equal parts.To prepare chicken roulades: Lay each flattened breast out on a large piece of plastic wrap. Divide the chicken mouse among the breasts and spread along the center of each. Now make a ribbon of olive puree mixture across chicken mousse on each breast. Roll up each breast and wrap securely in kitchen paper. Bring a large pot of water to a boil. Place the chicken breasts on a rack or the upper basket of a steamer so that they do not touch the water. Steam for 10 to 15 minutes.To serve: Slice each zucchini into thin vertical strips, leaving them attached at the stem end. Divide tomato sauce between 4 serving plates. Slice each chicken roulade into 5 slices and arrange around outer edge of tomato sauce on each serving plate. Fan zucchini garnish on each plate and add a basil sprig. Lightly brush each chicken roulade slice with olive oil.