Preheat oven to 400°F.
Chop cauliflower into medium to large bite-size florets. Toss cauliflower with EVOO, salt and pepper to coat. Roast on a baking sheet until just barely tender but slightly crispy, about 15 minutes.
For the topping, in a large skillet over medium heat, add 2 tablespoons EVOO and butter. When butter melts, add the garlic, lemon zest and parsley, and cook for 1-2 minutes or until fragrant. Add breadcrumbs and toast until golden. Transfer to a bowl and reserve.
Meanwhile, make the sauce: Melt butter in a saucepot over medium heat. Stir in the flour and cook a few minutes, stirring constantly, until it starts to smell slightly nutty. Stir in milk, salt, pepper and nutmeg, and bring to a simmer. Remove from heat and reserve.