Position an oven rack in the center of the oven, set a rimmed baking sheet on it and preheat to 450 degrees F.
Dry the broccoli and cauliflower as well as possible between paper towels. Melt 2 tablespoons of the butter in a large bowl in the microwave.
Add the vegetables and oil to the butter, and toss to coat. Carefully remove the preheated baking sheet from the oven, and spread the vegetables on it in an even layer; sprinkle all over with 1 teaspoon salt and 1/4 teaspoon pepper. Roast until the vegetables are dark brown on the underside and in spots around the edges, about 30 minutes.
Meanwhile, heat the remaining 1 tablespoon butter in a medium skillet over medium heat. Add the garlic, and cook, stirring constantly, until lightly golden, 1 to 2 minutes. Add the breadcrumbs, and stir until toasted and golden brown, 2 to 3 minutes. Remove from the heat, and stir in the parsley, lemon zest and 1/2 teaspoon salt.
Transfer the vegetables to a serving platter, and sprinkle with the lemon-garlic breadcrumbs.
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