Preheat the oven to 425 degrees F and line a rimmed baking sheet with parchment paper. Place the fennel on the prepared baking sheet and toss with with 1 tablespoon olive oil and 1/2 teaspoon salt. Move to one side of the baking sheet and roast until the fennel begins to soften, about 20 minutes.
Remove from the oven and place the crumbled sausage in an even layer in the center of the baking sheet. In a medium bowl, toss the broccoli rabe with the sliced garlic, red pepper flakes, remaining 2 tablespoons of olive oil and 1/2 teaspoon salt and spread into an even layer on the empty side of the pan.
Roast until the fennel and broccoli rabe are tender and charred in spots and the sausage is completely cooked through, about 15 minutes more. Toss vegetables and sausage with the accumulated juices on the tray before serving.
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