Bring 4 qt. salted water to a boil in a large Dutch oven over high heat. Add cauliflower; cook just until crisp-tender, stirring often. Drain; rinse under cold water. Let cool in colander.
Melt 3 Tbsp. butter in a large skillet over medium heat; add green onions and garlic, and sauté 3 minutes. Whisk in flour until smooth. Cook 1 minute, whisking constantly. Gradually whisk in milk and cream; cook over medium heat, whisking constantly, until mixture is thickened and bubbly. Stir in 1/2 tsp. each salt and nutmeg and 1/4 tsp. each pepper. Add cheese, stirring until cheese melts. Remove from heat; add cauliflower, stirring to coat well. Spoon cauliflower into a greased 13" x 9" baking dish.
Combine breadcrumbs and next 3 ingredients in a small bowl; sprinkle over cauliflower. Drizzle with melted butter. Bake, uncovered, at 400° for 35 minutes or until browned and bubbly.
*To make 1 1/2 cups homemade breadcrumbs, place 3 slices bread, torn, in a mini chopper. Cover and pulse just until you have fine crumbs.
Note: Farmhouse Cheddar gets its distinction, in part, because it's made on a farmer's property, using only milk from his cows. We tested with Keen's.