Place the salmon in the freezer for about 15 minutes while you prep the rest of the ingredients. The fish will be much easier to cut if it is very cold.
Remove the tarragon leaves from stem and smash into a paste, using mortar and pestle or a mini food processor. Combine with about 1 Tbsp. olive oil and reserve.
Dice the salmon in ¼ in. or smaller cubes. Combine with sliced scallions and lemon zest in a small bowl. Dress with about a ½ tsp. of olive oil, sea salt and pepper, to taste.
Form the tartare. If using a ring mold, start with the avocado base. Arrange slices of avocado to cover the bottom. Sprinkle the avocado with a little lemon juice to avoid discoloration and season lightly with salt. Spread the minced olives over the avocado, and top with the salmon mixture. I used a squat mason jar, lined with plastic wrap (helps get it out in one piece) and turned it upside-down to unmold it, so I worked in reverse, packing the salmon in first, then the olives, ending with the avocado. Once you’ve filled your mold, refrigerate for 15 minutes or longer to help it firm up and hold together.
Remove ring mold or invert jar onto a plate. Smear the tarragon oil across the plate or serve in a spoon to use as a condiment with the tartare.