Immediately afterward, toss the cauliflower with 2 tablespoons olive oil on your lined baking sheet. Sprinkle with salt and some freshly ground black pepper and arrange the cauliflower in a single layer on the pan. Roast on the upper rack until the florets are tender and the edges are deeply caramelized, tossing halfway. This took the full hour for me, but start checking the cauliflower for doneness around 40 minutes.
Add 4 cups broth (or 3 cups broth and 1 cup water), cover, and bring to a boil over medium-high heat. Remove from heat and stir in the rice. Cover the pot and bake on the lower rack until rice is tender and cooked through, about 65 to 70 minutes. It will seem pretty dry when you take off the lid, but don't worry!
Heat the olive oil in a medium Dutch oven over medium heat until shimmering. Add onion and a pinch of salt. Cook, stirring occasionally, until softened and turning translucent, about 5 minutes. Add the minced garlic and cook until the garlic is fragrant, 1 to 2 minutes.
Place your oven racks in the lower third and upper third positions. Preheat oven to 375 degrees. Line a large, rimmed baking sheet with parchment paper.