Cucumber Salad: Using a mandoline or a sharp knife, thinly slice the cucumbers, place in a colander and toss with about 1/4-1/2 tsp kosher salt per cucumber. Let sit and drain over the sink or a bowl for about 15 minutes. Cucumbers should give out some liquid and soften a bit. Meanwhile, make the sauce and cook the salmon.
Miso-Tahini Sauce: In a small bowl, mash together the miso and tahini with a fork. Stir in the sesame oil until smooth. Add a bit more if needed. Splash in about a tsp of room temp / warm water (or more) to thin out the mixture.
Add in the rice vinegar and continue to stir/whisk. If desired, drizzle in more water by the teaspoon to thin out the tahini mixture. Keep adding small amounts of water and stir until the sauce is smooth, well-combined, and at your desired consistency. Taste and add in more rice vinegar if you'd like the sauce to be more tangy.
Combine the crushed garlic with the tahini mixture. Taste and adjust seasoning. Add a bit of fish sauce (or just plain salt) if you need the sauce to be a bit saltier. Set aside as you cook the salmon.
Pan-Seared Salmon: Cut the salmon into 1/2 inch slices--you should have about 4-6 pieces. In a cast iron, carbon steel, or nonstick pan, heat the neutral oil until it's almost smoking.
When the oil is ready, sear the salmon for about 2-3 minutes on one side until golden, pressing the fish down occasionally. Then carefully flip and cook the other side for another 30 seconds-1 min. The salmon cooks quickly because the slices are relatively thin. Work in batches, if necessary, because you don't want to crowd the pan and cause the fish to steam instead. When each piece of salmon is done cooking, remove from heat and transfer onto serving plates.
Divide the cucumber amongst the plates. Spoon the miso-tahini sauce all over the fish and salad. Top everything with scallions, toasted sesame seeds, and a couple pinches of shichimi togarashi.