Preheat oven to 400 degrees F (200 degrees C).
Cut the Butter (2 tablespoon) into 1/4" dices. In a small bowl combine the All-Purpose Flour (1/3 cup), butter, and Granulated Sugar (1 tablespoon). Use fingertips to blend the ingredients together until they form small crumbles and hold together when you pinch some between your thumb and forefinger.
Grate the Parmesan Cheese (1/4 cup). Mince the Sage Leaves (4). Add the parmesan and sage and toss to combine. Set aside.
Slice the Cremini Mushroom (8 ounce). Heat 1 tablespoon of the Olive Oil (3 tablespoon). Add the mushrooms and sauté until they've given up their liquid and begin to brown, 5 to 6 minutes, stirring occasionally.
Mince the Garlic (3 clove). Chop the Fresh Sage (1 teaspoon). Add garlic and sage and cook for 30 to 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.
Peel and thinly slice the Onion (1). Cut the Cauliflower (4 cup) into bite sized florets. In a large bowl combine the cauliflower and onions. Add the remaining 2 tablespoons of olive oil, Kosher Salt (1/2 teaspoon) and Ground White Pepper (1/4 teaspoon) and toss to coat.
Add the mushrooms and Parmesan Cheese (1/4 cup) 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 Vegetable Broth (1/2 cup) and Cream (1/4 cup) 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 Tomato (1 cup). Bake for an additional 15 to 20 minutes until tomatoes pop and slump.
Chop the Fresh Parsley (to taste). While gratin is still warm sprinkle with parsley, a few Sage Leaves (to taste) and Lemon (1/4 teaspoon).