Preheat the oven to 375 degrees F (190 degrees C).

Bring a large pot of salted water to a boil. Add cauliflower, carrots, and celery and cook until tender-crisp, about 7 minutes. Drain and set vegetables aside. Return pot to the stove.

Melt butter in the pot over medium heat. Add onion and garlic and cook until onions are softened, about 4 minutes. Stir in flour, mustard, and black pepper. Cook and stir for 1 minute.

Gradually stir chicken stock into the pot; bring to a boil. Reduce heat and simmer until thickened, about 2 minutes, stirring frequently. Stir in milk. Add ham, cauliflower, carrots, and celery. Spoon mixture into a shallow 10-cup casserole dish.

Combine cracker crumbs, Edam cheese, almonds, and Parmesan cheese. Sprinkle over casserole.

Bake, uncovered, in the preheated oven for 30 to 40 minutes.