Rinse the cauliflower, broccoli, and onion. Slice them all evenly. Sprinkle the garlic salt over the vegetables. Toss lightly and place them in a covered container in the refrigerator. Let sit one hour.
Drain the cauliflower/broccoli mixture fully; mix in the sliced mushrooms.
Mix together the salad dressing and milk. Pour over the vegetable mixture. Mix lightly.