Position racks in the upper and lower thirds of the oven and preheat to 475 degrees F. Toss the cauliflower florets with 1 tablespoon olive oil on a baking sheet. Spread in a single layer and season with salt and pepper. Roast on the upper oven rack, 5 minutes. Add the almonds and roast until the cauliflower is tender, about 15 more minutes.
Meanwhile, heat the remaining 2 tablespoons olive oil in a Dutch oven over high heat. Add the onion and cauliflower stems and cook, stirring, until the onion is translucent, about 3 minutes. Add the rice and garlic and cook, stirring, 2 minutes. Add the wine and cook, stirring, until evaporated, 1 to 2 minutes. Add the broth, 2 cups water and 1 teaspoon salt; cover and bring to a boil. Transfer the pot to the lower oven rack and bake until the rice is tender, about 15 minutes.
Remove the rice from the oven and add the butter, fontina and parsley, stirring vigorously until the risotto is creamy, 1 to 2 minutes. Divide among bowls and top with the roasted cauliflower and almonds.