For the salmon: Preheat the broiler on high. Line a rimmed baking sheet with foil and brush it lightly with oil.
Whisk the sake, miso, sugar and sesame oil in a large, microwave-safe bowl; microwave until just simmering, about 40 seconds. Alternatively, whisk the mixture in a small saucepan over medium-low heat until just simmering, stirring occasionally to combine and to dissolve the sugar and miso. Cool to warm room temperature, whisking occasionally.
Toss the radishes with the remaining 2 teaspoons of vegetable oil, a tablespoon of the miso sauce and a pinch of salt. Arrange around the edges of the prepared baking sheet.
Coat the salmon in the miso sauce then arrange the fillets in the center of the baking sheet and pour the remaining miso sauce over top. Broil 4 to 6 inches from the element, rotating the pan occasionally, until the radishes are crisp tender and browned and the fish is just cooked through and golden brown in spots, about 6 minutes.
To make the salad dressing, whisk the miso and rice vinegar in a large bowl to break up the miso. Next add the lime juice, coconut aminos, sesame oil and sesame seeds. While whisking, drizzle in the vegetable oil. Add the sliced cucumbers and spring mix and toss to coat.
To serve, plate some of the salad and top with broiled salmon and radishes. Garnish with the scallions and serve!