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I love a nice seared piece of tuna that is rare on the inside, but cooking it at a very low temperature with sous vide ensures that it is evenly cooked throughout. It also softens the fish slightly, making it even more delicious. You aren't trying to cook the tuna much so stick to a low temperature, I usually do 110°F (43.3°C) until it is just heated through.
Preheat a water bath to 110°F (43.3°C).
Salt and pepper the tuna then place in a sous vide bag. Add the olive oil then seal. Cook until heated through, 20 to 60 minutes.
Whisk together the peanut oil, sesame oil, lime juice, vinegar, soy sauce and fish sauce. Whisk in the ginger, shallot, and serrano chile. Toss the remaining ingredients with the dressing.
Heat the canola oil in a pan over medium to medium-high heat.
Combine the sesame seeds in a bowl. Remove the tuna from the bag, pat dry, and coat with the sesame seeds. Sear on the top and bottom until the sesame seeds become fragrant and begin to toast. Remove from the heat and slice into 1/2" strips (13mm).
Place several slices of the tuna on a plate. Add the avocado salad to the side then serve.
A good way to make this dish fancier is to make an avocado mousse. Remove the flesh from 2 avocados and blend them with half of a serrano pepper and 1 cup of heavy cream. Strain the mixture into a whipping siphon, seal and charge it. It will last in the refrigerator for a day or two. The mousse can be dispensed on top of the tuna steaks when serving.