Preheat oven to 400°.
Place bread slices in a food processor, and process until fine crumbs measure 3 cups. Set aside.
Cook cauliflower in boiling water 15 minutes or until tender; drain.
Melt 1 tablespoon butter in a large nonstick skillet over medium heat. Add leek and 3 garlic cloves; cook 7 minutes or until tender, stirring frequently. Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and milk, stirring with a whisk; add to pan. Bring to a simmer; cook 2 minutes or until thick, stirring constantly. Stir in 3/4 teaspoon salt, cheese, and pepper. Remove from heat; stir in cauliflower. Spoon cauliflower mixture into a 13 x 9-inch baking dish coated with cooking spray.
Melt remaining 1 teaspoon butter in a small skillet over medium heat. Add remaining 3 garlic cloves; cook 30 seconds or until lightly browned, stirring constantly. Remove from heat. Combine breadcrumbs, remaining 1/4 teaspoon salt, parsley, and thyme. Drizzle with garlic-butter mixture; toss to combine. Sprinkle breadcrumb mixture evenly over cauliflower mixture. Bake at 400° for 30 minutes or until bubbly and browned.