For the mint mayonnaise: Place the mayonnaise, mint and lime juice in a bowl and stir to incorporate. Season with salt. Cover with plastic wrap and refrigerate until assembling the lobster rolls.
For the carrot ginger sauce: Place the carrots, ginger, 2 1/2 cups water and salt to taste in a pot. Bring to a simmer and cook until the carrots are tender and the liquid has reduced by half, 20 to 30 minutes.
Place the carrot mixture in a food processor and pulse a few times to puree the carrots and ginger. Place the carrot mixture in a fine sieve over a small pot. Press all the liquid through the sieve into the pot. Place the pot on the stove, bring up to a simmer and whisk in 1 1/2 to 2 tablespoons of the cornstarch and water mixture. Simmer until the liquid thickens and coats the back of a spoon. Remove from the heat. Whisk in the olive oil when cool.
For the napa cabbage lobster rolls: Place the napa cabbage leaves on a clean, dry surface. Divide the chopped lobster meat along the stem end of each cabbage leaf. Season the lobster with salt. Spoon some mint mayonnaise over the lobster. Place the celery and green onions along either side of the lobster. Spoon 1/2 teaspoon sambal oelek over the filling. Place 2 segments of grapefruit in the cabbage roll, followed by pieces of cilantro. Proceed to roll the cabbage to form a roll. Cut the rolls in half.
Spoon some carrot ginger sauce on a plate. Place the napa cabbage lobster rolls in the center of the plate. Garnish with diced avocado and sesame seeds.