In a 10- by 15-inch baking pan, mix broccoli, oil, and 1/8 teaspoon salt. Spread in a single layer.
Bake in a 450° oven, stirring occasionally with a wide spatula, until broccoli is browned and tender when pierced, 20 to 25 minutes.
Meanwhile, in a bowl, mix balsamic vinegar and mustard. Add warm broccoli, walnuts, and frisée, if using, and mix to coat. Add salt and pepper to taste.
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