Because they use modernist ingredients, these amounts are given in metric by weight. For more information on how to measure modernist ingredients check out this article.
Combine all ingredients in a pot and bring to a boil. Stir well. Let simmer for 5 minutes.
Add the pears, jalapeño, and ginger to the whipping siphon. Strain in the pickling liquid. Seal the siphon and charge. Let the siphon sit for 20 to 30 minutes then vent and remove the pears and pickling liquid from it.
The pickled pears can be stored in the pickling liquid in the refrigerator for several weeks.
Place 150g of pickled pears and pickling liquid into a container. Add water and blend until smooth. If it is not thin enough, add some more water. Add the xanthan gum and blend well.
Add the remaining 150 grams of just pears and blend lightly, leaving some chunks in the relish.
Combine all ingredients in a bowl and blend well. The Asian accent sauce will keep in an airtight container placed in the refrigerator for several weeks.
Mix the spices together in a small bowl.
Cut the tuna into 1" (25mm) wide rectangles that are as long as the tuna steak. Salt and pepper the tuna rectangles then coat with the spice mix.
Heat oil in a pan over medium to medium-high heat. Sear the tuna on each side until just browned, about 30 to 60 seconds per side, then remove from the heat.
Cut each tuna rectangle crosswise into pieces 0.5" to 0.75" (12mm to 19mm) long. Place a piece on a serving dish, top with some pickled pear relish, a dot of the Asian accent sauce, and some basil leaves.