Instructions

• First, prepare the agar or gelatin. Cut the agar threads into half-inch pieces and pack them to the ¾ line in a measuring cup. Soak for 15 minutes or overnight. Then cook in the water, stirring with a metal spoon, for 5 to 10 minutes until there are no visible dots or thread residue when you dip a clean metal spoon into the mixture. This ensures that the agar (or gelatin) mixes entirely and sets. Keep it warm, or reserve and reheat to melt the agar. If using powdered gelatin, follow the package directions.
• Next, prepare the tomatoes. Add 32 32-ounce can of chopped tomatoes in their juices to a large cooking pot. Squeeze the tomatoes with your hands to macerate the pieces. Add diced onion, lemon, sugar, peppercorns, cloves, bay leaf, and salt. Bring to a boil over high heat, stirring occasionally, then reduce the heat to medium and simmer for about 20 minutes. Strain the mixture through a fine mesh over a bowl, then save the juice in the pot. The leftover pulp can be used in recipes later.
• Then, prepare the other ingredients. Make the hard-boiled eggs. Cool, peel, and thinly slice. Slice olives. Lightly sauté peeled shrimp and chill until ready to assemble.
• Now it's time to assemble and chill. Add ½ cup of warm fish broth, shellfish broth (such as mussel or clam), or chicken broth to the tomato liquid and bring it back to a boil. Remove from heat and mix the agar (or gelatin) into the hot broth. Stir in minced chives or tarragon. Add the white vinegar and mix. Decoratively arrange egg and olive slices in the mold. Slowly ladle in a shallow layer of the tomato broth, then add shrimp evenly between ladles. Take your time here. The shrimp and other ingredients will float if not done carefully. It can be a complicated process to keep the shrimp distributed evenly, so make sure to wait for 5 to 10 minutes between each layer of aspic. It should be just tacky to touch, not solid, or layers will come apart. Lightly scoring the aspic between layers with a fork helps them to adhere to each other. Repeat until the ring mold is complete. Skipping these steps or not waiting long enough in between layers will cause the shrimp to float to the surface. It’s tricky, but a little patience makes it less intimidating. Chill in the refrigerator for at least 6 hours or overnight.
• Next, prepare the pesto leaves. Pour a ¼ cup of store-bought pesto sauce into a mixing bowl. Soften one wheel of Boursin cheese. Mix with ½ cup of prepared agar. Pour the mixture into small, silicone molds (such as the leaf design used here) or onto a baking sheet and chill for at least 4 hours. Use cookie cutters once gelled.  
• Finally, invert the raised platter over the mold and carefully invert both. If the mold doesn’t release, set it in a hot water bath for about 45 seconds. Place unmolded pesto cheese leaves around the tomato ring. Bring to the table and slice for guests. Serve on a bed of lettuce greens, drizzled with oil and lemon, and garnish with pesto cheese.