Wash, trim, and boil the cauliflower in salted water until very tender. Drain well and mash with a fork until it's almost a puree consistency. Mix in the drained, flaked tuna.
Finely chop the chives and garlic, then add them to the cauliflower mixture. Stir in the grated cheese and combine the mixture with the lightly beaten egg. Add a few tablespoons of milk to bind the mixture and season with freshly ground black pepper.
Refrigerate the mixture for 30 minutes. Form into small croquettes, roll in breadcrumbs, and fry in hot oil until golden brown on all sides. Drain on paper towels and serve hot.