In a small bowl combine the flour, sugar and butter. Use your fingertips to blend the ingredients together until they form small crumbles and holds together when you pinch some between your thumb and forefinger. Add the parmesan and sage and toss to combine. Set aside.
Heat 1 tablespoon olive oil in a medium saucepan over medium heat. Add the mushrooms and sauté until they've given up their liquid and begin to brown, 5-6 minutes, stirring occasionally. Add garlic and sage and cook for 30-40 seconds until fragrant. Remove pan from heat and cover with a tight fitting lid - to steam and soften the sage and garlic, without browning. Set aside.
In a large bowl combine the cauliflower and onions. Add the remaining 2 tablespoons olive oil, salt and pepper and toss to coat. Add the mushrooms and parmesan cheese and toss to distribute the ingredients evenly. Transfer the vegetables into a small square or rectangular casserole dish, about 8" x 8".
Stir together the broth and cream and pour over the vegetables. Sprinkle the crumble evenly over the gratin. Cover the gratin with tin foil and bake for 45 minutes, until cauliflower and onions are very tender. Remove the foil and dot the top of the gratin with cherry tomatoes. Bake for an additional 15-20 minutes until tomatoes pop and slump.
While gratin is still warm sprinkle with parsley, a few small sage leaves and lemon zest. Serve.