For the noodles and dressing: Begin by making the dressing. In a large bowl, combine the miso, soy sauce, 1/4 cup hot water, mirin, canola oil, rice vinegar and sesame oil. Whisk together well and set aside.
In a large pot of salted boiling water, blanch the sugar snap peas until bright green and just tender, 10 to 15 seconds. Set aside. Add the soba noodles to the hot water and cook for 2 to 3 minutes. Then strain and rinse under cold running water (this will stop the cooking process and gives the soba noodles their distinctive springy texture). Toss the noodles in the dressing and fold in the sugar snap peas and julienned carrot. Set aside until ready to serve.
For the seared ahi tuna: In a large mixing bowl, make a quick marinade of the canola oil, sweet soy sauce, yuzu, sesame oil and ginger. Whisk together to combine. Carefully cut the tuna into 1 1/2-inch cubes and place in the marinade and toss to coat well (it will be a very light coating/marinade as the tuna shouldn't be too wet when searing).
Thread 4 pieces of tuna per bamboo skewer (4 cubes per skewer, 8 skewers total). Mix the sesame seeds together and spread out on a flat platter. Take each skewer and press the tuna into the sesame seed mixture so it is well coated on all 4 sides.
Heat a flat grill plate to high heat. Brush lightly with some oil to clean the grates and create a nonstick surface. Grill the tuna skewers just a few seconds on each of the 4 sides so the sesame seeds are toasted and crunchy and the tuna is rare in the middle, 30 to 35 seconds.
Serve the tuna skewers on a bed of the dressed soba noodles and garnish with some sliced scallions.