Preheat the oven to 325° and line a baking sheet with foil. Spread 6 tablespoons of the Dijon mustard on the prepared baking sheet a scant 1/4 inch thick. Bake the mustard for 10 to 12 minutes, until a crust forms on the surface and the mustard feels slightly firm.
In a blender, combine the baked mustard, lemon juice, vinegar, capers and brine with the remaining 2 tablespoons of Dijon mustard and blend until smooth. With the blender on, drizzle in the olive oil and canola oil until thick. Transfer the dressing to a jar; stir in the whole-grain mustard. Season with salt and serve.