Remove leaves from 1 artichoke heart; pat dry, and reserve. Thinly slice remaining artichokes; pat dry.
Preheat oven to 400 degrees. Melt 4 tablespoons butter in a saucepan over medium heat. Add flour, and cook, stirring constantly, for 2 minutes. Whisk in milk, and bring to a boil. Add salt, pepper, and cayenne, and simmer until thickened, about 2 minutes. Remove from heat, and stir in cheeses.
Melt remaining 2 tablespoons butter in a skillet over medium-high heat. Add onion, and cook 3 minutes. Add thyme, garlic, and sliced artichokes, and cook for 3 minutes. Add to cheese mixture along with zest. Transfer to an 8-cup baking dish. Top with reserved artichoke leaves. Sprinkle with breadcrumbs.
Bake until golden and bubbling, about 15 minutes. Let cool slightly. Garnish with thyme leaves. Serve with crudites.