Pre-heat oven to high broil and place oven-proof plate or cookie sheet in middle of oven.
In a bowl, combine 1/4 cup Dijon mustard with panko, olive oil and scallion whites. Season trout and spread seasoned panko mixture on top of trout. Meanwhile, make vinaigrette by mixing lemon juice with ponzu and 1 tablespoon Dijon mustard. Whisk in the grapeseed oil. Season and check for flavor.
Add diced cucumbers and quinoa. Check for flavor and season.
To plate, arrange overlapping circle of cucumber coins, then spoon quinoa salad in center. Top with trout, drizzle extra vinaigrette around plate and garnish with reserved scallion greens.