Make the Everyday Gochujang Sauce:In a glass jar, stir together the gochujang, vinegar, maple syrup, garlic, soy sauce, and sesame seeds until fully incorporated and smooth. Use right away, or cover and refrigerate for up to 5 days.Cook the salmon:Preheat oven to 325 degrees. Line rimmed sheet pan with foil. Place salmon on top. Pat dry with paper towels. Season with salt and pepper.Using a silicone brush or back of spoon, spread gochujang sauce evenly over top of salmon. Bake until thickest part of salmon reaches 135 degrees on instant-read thermometer, about 15-20 minutes. Remove from oven. Heat broiler.Make the cucumber apple salad:In medium bowl, stir together cucumber, mint, vinegar, sugar, and salt. Core, quarter, and thinly slice apple. Stir into bowl. Refrigerate until time to serve.Make the spicy mayonnaise:In small bowl, combine mayonnaise with gochujang sauce. Stir to mix well.Serve:Broil salmon until glaze is sticky, about 2 minutes.On large serving platter, transfer salmon. Garnish with green onions.Serve alongside spicy mayonnaise, cucumber apple salad, lettuce leaves, and individual bowls of rice, if desired.