In the same pan, with the remaining oil, sear tuna steak over medium-high heat for 10-15 seconds on each side. Transfer to a cutting board, and cut in half horizontally. Then cut each large horizontal piece into six pieces with one side seared.
To assemble the stackers, combine mayonnaise and pesto; spread 1/2 teaspoon sauce on top of each cucumber and each zucchini slice and add 1 cheese slice.
Top the cucumber stackers with tuna, seared side down and top with a sage leaf. Secure each stackers with a toothpick and place on one platter.
Top the zucchini stackers with kielbasa slices and top with oregano leaves. Secure each stackers with a toothpick and place on another platter.