Preheat oven to 375 degrees Fahrenheit. Heat a large dutch oven on medium high. Add 2 tablespoons of olive oil along with the sausage and onions. Cook, breaking up the sausage as you go, about 10 minutes, adding garlic halfway through. Remove mixture from pan and set aside.
Add cauliflower to the reheated pot and brown on both sides, about 2 to 3 minutes, seasoning to taste with salt and freshly ground black pepper.
Add the broken up tomatoes with their liquid, along with the cooked sausage mixture. Bring simmer and adjust for seasoning. Mix in half the parsley and spread the mixture in to a 13x9 baking dish.
In a small bowl, mix together the panko breadcrumbs, Parmesan cheese, and remaining parsley. Sprinkle the mixture evenly over the top of the sausage and cauliflower and bake until golden brown on top and crispy on top, about 30 minutes. Remove from the oven, let sit for 5 to 10 minutes, then serve.