Preheat the oven to 475 degrees . On a rimmed baking sheet, toss the vegetables with the oil; season with salt and pepper, arrange in a single layer and roast until golden and tender, stirring once, 15 to 18 minutes.
Meanwhile, in a medium saucepan, bring 5 cups water to a simmer. In a large saucepan, melt 2 tbsp. butter over medium. Add the onion. Season and cook, stirring, until translucent, 3 minutes. Add the barley to the onions; cook until coated, stirring constantly, 1 minute. Add the tomato paste, fennel seeds and 1 cup simmering water. Reduce the heat to medium-low and simmer, stirring often, until almost all of the water is absorbed. Repeat with the remaining water, stirring in 1 cup at a time, until the barley is tender, about 15 minutes.
Stir the roasted vegetables, remaining butter and 1/4 cup cheese into the risotto over medium; season. Divide among bowls; sprinkle with the rosemary and remaining cheese.