► Heat oven to 400°F. Grease 2-quart baking dish (can be rectangular, square, or oval).
► Fill large pot with water and bring to boil. Salt generously and add cauliflower florets, cooking until florets have slightly lost their crisp but are still firm, about 3 to 4 minutes. Drain and set aside.
► Meanwhile, melt butter in a large pot or skillet over medium heat. Add leeks and cook until softened, about 5 to 6 minutes. Stir in garlic, thyme, and ½ teaspoon salt and cook for another 2 to 3 minutes.
► Add flour, stir to incorporate it into leeks, and cook for a minute or two. Slowly stir in milk, breaking up clumps as you go. Bring to a slight simmer over low heat and cook until sauce has thickened, stirring frequently to keep bottom from burning.
►Once sauce is thick enough to coat back of a spoon, stir in nutmeg, black pepper, 1 teaspoon salt, and 1 cup grated Pecorino Sardo. Once cheese has melted, add cauliflower and mix until fully coated.
►Transfer cheesy cauliflower mixture to baking dish and spread evenly. In a small bowl, combine breadcrumbs with remaining Pecorino Sardo. Drizzle with olive oil and season with salt and pepper to taste.
►Sprinkle breadcrumbs over the gratin and bake for 40 to 45 minutes, or until breadcrumbs are golden brown and crisp and sides are bubbling. Allow to slightly cool before serving.