Preheat oven to 425°F with rack in top third position. Place broccoli and shallots on a large rimmed baking sheet. Drizzle with 1 tablespoon oil, tossing until evenly coated; arrange in an even layer. Roast until the broccoli is bright green and browned in spots, 18 to 20 minutes. Let cool slightly, about 5 minutes.
Meanwhile, whisk 1 teaspoon lemon zest, 1½ tablespoons lemon juice, minced garlic, ½ teaspoon mustard and ¼ teaspoon each salt and pepper together in a small bowl. Whisking continuously, drizzle in the remaining 3 tablespoons oil until the mixture is emulsified, 2 to 3 minutes.
Transfer the broccoli mixture to a large bowl. Pour in the vinaigrette; toss well to coat. Sprinkle with ½ cup feta and 2 tablespoons each sunflower seeds and cranberries.
