To prepare vinaigrette: Combine 1/4 cup oil, vinegar, mustard, salt and pepper in a mini food processor or blender; process until creamy.
To prepare salad: Place a piece of parchment or wax paper near your stove. Heat 1 tablespoon oil and butter in a medium skillet over medium heat. Add walnuts and cook, stirring, for 2 minutes. Add salt and pepper and drizzle in maple syrup. Cook, stirring, until the nuts are well coated and have begun to caramelize, 3 to 5 minutes. Transfer to the paper, spooning any remaining syrup over them. Separate the nuts while they’re still warm. Let stand until cool, about 5 minutes.
Place cabbage and scallions in a large bowl. Toss with the vinaigrette. Serve topped with blue cheese and the walnuts.