Preheat the oven to 400°F (175°C). Set a rack to the top third of the oven. Line a baking sheet with parchment paper, brush the parchment with 1-1/2 teaspoons of the olive oil.
In a large pot, combine 1 inch of water and the cauliflower florets. Bring to a boil, cover with a lid, and steam until the cauliflower is very tender when pierced with a fork, about 7 minutes. Drain and let cool.
In a food processor fitted with the "S" blade, process the cooled cauliflower to rice-size pieces, working in batches if necessary. Transfer the cauliflower rice to the center of a thin, clean, dishtowel. Roll up dish towel with the cauliflower inside, then twist it to squeeze out any excess moisture.
In a large mixing bowl, use a spoon or rubber spatula to mix together the cauliflower rice with 1/2 cup of the mozzarella and the Parmesan, dried oregano, garlic powder, eggs, 1/2 teaspoon salt, and 1/4 teaspoon pepper until the pizza crust “dough” is smooth and like thick pancake batter. Transfer the “dough” to the center of the prepared baking sheet, then use a rubber spatula or clean hands to pat it into a flat circle with a 10- to 11-inch diameter. Brush the dough with 1-1/2 teaspoons of the olive oil and bake for 30 minutes.
Place a piece of parchment paper over the baked dough (pizza crust), carefully flip the crust upside-down, then peel off the parchment paper. Brush the top of the crust with 1-1/2 teaspoons of the olive oil, return the crust to the baking sheet and cook until the crust is evenly golden brown, firm, and slightly crispy, about 25 minutes more.
Meanwhile, add 1/2 inch of water to a large sauté pan with a lid and bring it to a boil over high heat. Add the broccoli rabe, reduce the heat to medium-high, cover the pan, and let steam for 3 minutes. Drain and let cool.
Discard the water from the sauté pan, set the pan over medium heat, then, when the pan is dry, add the remaining 1-1/2 tablespoons olive oil. When the oil is shimmering, add the sliced garlic and let sizzle and get a bit golden, about 1 minute. Add the mushrooms and sausage and cook, stirring often and breaking up the sausage, until the sausage is cooked through, about 7 minutes. Set aside.
Chop the cooled broccoli rabe into 1-inch pieces. Add it to the sausage and mushrooms, then season to taste with salt and pepper.
Drizzle the remaining 1 tablespoon olive oil over the pizza crust. Spread the sausage mixture evenly on the crust. Top with the remaining 1/2 cup mozzarella and the fontina cheese, then bake until the cheese is bubbly and golden, 10 to 12 minutes. Sprinkle with oregano leaves and serve.