Combine soy sauce, sesame oil, and corn starch in a small bowl. Whisk well and set aside.
Pat Ahi Tuna dry and season with salt and pepper. Heat a pan on medium-high heat and add olive oil. When olive oil is shimmering, add Ahi Tuna and sear for 2 minutes per side, until the outside is seared, but the inside is rare. Remove from skillet and set aside.
Slice Ahi Tuna thinly. Assemble lettuce cups by adding sliced Ahi Tuna, shredded carrots, cucumber slices, and avocado slices to butter lettuce pieces.